Basically, everything you could do, the Eldar could do better. In all of the expansions for the original Dawn of War, the Eldar are ridiculously powerful (in the first game they merely had an advantage because they were good at taking down heavy infantry, which both Marines and Chaos relied heavily on).For this reason, later expansions made the Whirlwind have a cap of 1 max. Any enemy armoured forces and even super units can easily be dealt by Assault Terminators and Tactical Squads. This meant that Space Marines can easily clean the map of enemy infantry by tying them down with cheap but mighty Assault Marines and then completely murdering them with constant missile bombardments from multiple Whirlwinds. Their forces gained significant health and damage buffs, especially for the Whirlwind tanks and Assault Marines. The Space Marines in the Winter Assault expansion became gods of war.Oh, and for a few seconds after the commander's death, the drones can still use the jumppack ability. Meaning that you can send the commander in the middle of an enemy base or mob and continuously revive him, for free, instantly, and it resets all his cooldowns, as long as the drones are alive (and while the drones don't reinforce instantly, they're also free). The campaign commanders have a hilariously broken mechanic with their drone wargear: as it makes them act as a squad, the commander can be reinforced if he dies (like the IG's Command Squad).Just fly in the Barracudas through and you can destroy all the Chaos Shrines or Necron Monoliths and run down Vect's Dias of Destruction without your footsoldiers ever leaving your base. This is especially egregious when attacking strongholds because there are a number of missions where the terrain is supposed to force your army to run a gauntlet of enemy defense. They have huge amounts of armor and firepower and the cap is 5, you can basically send them in to level an enemy base while the rest of your army sits at home. While a skilled human player can usually adequately defend against them, the Tau Barracuda fighter-bombers from Soulstorm are extraordinarily broken in campaign mode.They also have early stealth units, which are effective since the enemy will not have many detector units so early in the game. The Tau in Dark Crusade, likely because they have very powerful ranged weaponry, making it difficult for the other units to close in.Soulstorm made it even more broken with the Essence Of The Deceiver ability, which turns him into a giant invincible god of deceit (who can summon a fake Monolith and mind control a squad).Which unit do you think is going to be sent into the base as a spotter? The icing on the cake is that once killed, the Necron Lord can be respawned where he died, all upgrades intact. The Necron Lord is a strong melee unit that can teleport and can be equipped with, say, a regenerative ability that reduces ranged damage, the ability to turn himself and surrounding units invisible, can resurrect said units en masse, and can turn into a giant invincible god of death.This works as well in multiplayer as it does in the campaign. Get one unit into the enemy base, teleport in all your Flayed Ones, and wail on the HQ.
The Necron can instantly teleport up to five squads of Flayed Ones any place not covered by Fog of War. In Dark Crusade, once you destroy an enemy HQ, they lose, regardless of any other units or buildings they have left.The Necron Lord himself may be considered overpowered since when fully upgraded, he can temporarily turn himself into an invulnerable C'tan.
They do not need requisition (the main resource of the game, Necrons will get a a boost to their build times instead when capturing strategic locations), the Necron Warriors are free (although they take a long time to build, and the more Warrior squads you have, the longer it takes to build them), as well as the resurrection ability (which means that destroyed units have a chance to come back to life, which can take the Necrons above the headcount limit).