The first thing you should know both playing a Canoptek Harvest as well as playing against it are that the Canoptek Scarabs are the key to its success. They should be focused down as quickly as possible, should you face it and be protected early on if you play it.
The Canoptek Harvest is deceptive. When I first put it together I fully anticipated that the Wraiths would be the big producers for the list. Just look at how terrible the stat line really is for Canoptek Scarabs! In addition, hasn't the internet not fed you a healthy diet of "OMG" about the Wraiths and how much "better" than terminators they are etc... To hear some tell it, the sky is falling. Here's a reality check: They don't have high WS, Rending is nice but highly unreliable in the amounts the Wraiths put out, and the Wraiths are NOT the priority target in this army!
Canoptek Wraiths are a moving cover screen to help ensure the Canoptek Scarabs safety in case you go second. Scarab 6+ saves won't stop anything and they will not yet have Reanimation from their attendant Canoptek Spyders if they cannot go first. The Harvest will want the opponent to focus on the Canoptek Wraiths because there is a ton of weaponry that can wreck the Canoptek Scarabs, but not so much the Wraiths. Anything that is STR 6 is removing entire Canoptek Scarab bases at a time. Even just a couple of assault cannons from Razorbacks can thoroughly ruin a Canoptek Scarab unit. In fact despite conventional wisdom, let me assure you that starting with fewer Canoptek Scarabs nd spawning the units up to size from there isn't necessarily a wise thing to do. In round one you will likely be able to pump 4 bases into the Scarab unit of your choice and one or two into the other units. Getting to a size of critical mass is important. Starting with eight Scarab bases in a unit, like I did, is expensive in points and dollars, but ultimately proved quite necessary against the very common things you see like Scatterbikes and Assault Cannons.
Caoptek Scarabs are absolutely terribad in close combat. As I experimented with the list, I learned that they are essentially like Fearless Gretchen. They lose round after round of combat most of the time and it became a running joke that "the Canoptek Scarabs lose combats until its enemy is dead". It's quite true! Even with 14 Scarab bases crashing into a Striking Scorpion squad, the Scarabs took far worse than they gave. Their numbers were huge and able to absorb the massive losses. From a board control perspective you simply won't find any unit that's better at it, though. That's the way the Canoptek Harvest REALLY works. It does not attempt to WIN combats, it just grinds you down eventually. Not even the Canoptek Wraiths really focus on the goal of winning combats although they can actually do it.
The goal is to get across and entrenched in close combat at mach speed as far forward as possible and cut off enemy movement completely; then to stay there, holding the enemy in place, while the Canoptek Spyders, Immortals and the Canoptek Wraiths remain free to score points for you! The Wraiths are far better for scoring points for you in BAO (Bay Area Open tournament) missions wherein you don't score objectives til both people have had a turn. Going second isn't nearly the curse then; and in other more traditional missions the Wraiths mobility is such a boon, ignoring terrain and flitting about where they are needed. Canoptek Wraiths are also strong for their ability to go to dangerous places on the board with much less worry of loss to shooting than other units will have. they can take hits that none of the other units in the army dare take.
The Canoptek Wraiths are desperately over powered by Gargantuans and Super Heavy Walkers. Consider that Scarabs and Wraiths need the same exact thing to hurt either one: a 6+. Given that is true, you'd probably rather have the volume of opportunities Scarabs provide (5 on the charge and a LOT more of them) than the lower number Wraiths give you (4 on the charge nd a lot less of them). The Whip coils are fine for trumping initiative but it doesn't magcally give them more attacks and losing Wraiths to Stomp is so damn painful that I don't even think there's a real choice between the Wraiths and Scarabs: Take the big stuff on with Scarabs and expect massive losses. The army in general is poorly suited to facing those things but if forced, your only real choice should be the Scarabs after they reach a suitably critical mass in size.
Wraiths do not put out enough damage nor have the wounds they need to really stand up to things like charging Meganobz and Paladins (though they will last longer than many would) unless they are very small Paladin/Meganobz squads. Whereas Scarabs can make up for the heinous losses they take through spawning and the Swarm rule wound allocation shenanigans, the Wraiths feel every loss to their number, so you must be cautious which combat they assist in. But when forced, bring all the Wraiths you can because Rending truly is unreliable but enough of it brought to bear on such units will still grind them down eventually.
Going first with a Canoptek Harvest army is crucial as I mentioned. While you certainly are not in immediate danger of losing if you can't go first, a canny enemy with the right tools can dice you up something fierce before you hit the start line and you will be thanking your stars you can spawn more Scarabs. This is not an army meant for null Deploymnt tactics. It is an absolutely predatory list. It must be attacking constantly to maximize it's usefulness.
Aside from the soft underbelly that the Scarab units provide to the canny enemy, and the Wraiths ineffectiveness against the big stuff relatively speaking, it has another problem that might not SEEM obvious at first: armor. cheap armor that the Harvest must chomp through is tough because unlike many armies, the Canoptek Harvest is focused on assault and really has no reliable ways of popping open vehicles UNTIL the armor is out of the way. So what happens in practice is that the Swarm get forced to eat the 35 point rhino AS its entire turn, which means you have spent a very considerable amount of points doing nothing but setting yourself up for flamers and blasts that do serous double damage to Scarabs. So a foe that uses these tools a fair amount will delight in the damage that can be visited upon this force simply by slowing their roll and wasting their energy on the mundane task of popping rhinos which other armies would have done with shooting.
The Canoptek harvest list I ran, while it does technically have some shooting, it doesn't get used much. The (completely ridiculous) ITC FAQ which some tournaments use cuts the balls off of the Canoptek Harvests main ability by changing the rules completely for how the Canoptek Spyders grant the special rule to their attendant units. It changes the rule to read that you don't grant the powers and they DONT last to the beginning of your next turn! Not awesome. So in ITC tournament that use the FAQ It forces the Canoptek Spyders to essentially move and run all the time to keep up with their very quick charges. The net result of this is that if you are in an ITC FAQ'd tournament, you will not find that this list shoots until the third round if you're doing it right, unless the enemy has foolishly walked into you. By then, so much of the enemy is tied down in combat that there's hardly an opportunity worth taking because Canoptek Spyder weapons are only 24". It limits the targets worth shooting a fair amount of the game. So while you might get lucky once in a while, in most matchups, don't expect to be doing much shooting at all.
With no Psyker phase and a very limited shooting phase, you can see that having to stop to kill 35 point rhinos could be quite the slow down. Add to this the Space Marine Battle company and its ability to take 600 points of vehicles for free? You can see that certain armies are going to pose a problem, The solution? Disorganized charges. The Canoptek Scarabs will be in great numbers if the enemies opts to stay in their metal cans longer. The disorganized charges, while they might not kill very many vehicles, will scare the enemy into getting out and fighting if you do it against a broad enough front. It actually takes a fair number of scarabs to reliably kill a vehicle now. In a disorganized charge it takes NINE Canoptek Scarabs to take down one Land Raider. Not as good as they used to be. Enemies wont want to be trapped and if they see three or four of their transports go up in a plume of smoke, they may well rethink staying inside too long and allowing those Scarab bases to multiply further the next round.
There will be a need to be cognizant of terrain. The first round of movement is critical. You want to choose a board side, when allowed, that gives your units a safe haven midboard at the end of round one so that you're not relying on 6+ saves for your Scarabs. Terrain DRAMATICALLY increases the threat of the Canoptek Harvest list I detail below. The significance of that cant be overstated.
You will want to use the Strategic Warlord Traits table. The Necron table is a complete waste of your time with this list.
You will want to start everything on the redline and get ready to roll. The added distance two separate units of Canoptek Spyders can make to the Scarabs movement is enormous. Unwary enemies will not generally expect a first turn charge but this army absolutely will do that if the enemy is not paying attention and with a little luck.
The Canoptek Spyders are almost impossible to hide or protect by any means other than target priority by the enemy and saturation up front. They will probably die in many games. However, the distraction they provide is valuable and while I'm not advocating carelessness, a certain callous disregard for their eventual loss is warranted. In the early going their presence is critical but as the game wears on you will outrun your Canoptek Spyders whether you play with the absurd ITC FAQ or not. It's at that point that the Canoptek Spyders will take the role of more traditional shooters (the Spyders that are actually left, that is). You'll enjoy their twin linked blasts late in the game, albeit again, overall they wont want to get too many shots off if its between that and offering Reanimation or Shred ability to units they must keep up with.
The Canoptek Harvest is a winning army. I've proven over many games now that it is to be feared. I can tell you that using the version I'm posting will not only be expensive but also very likely to end in victorys if its run by a savvy general who is aware ahead of time of its faults. The one army that proved consistently to defeat it were White Scars with all their relentless Grav Cannons they could fit into the army plus a pair of Storm Talons. Outside of that one match up difficulty, it has been more than impressive and even in those games both sides were decimated by games end. Give it a try. This is the friendlier version without the Decurion element to it, but it can be made meaner.