There are a couple of units the Eldar arsenal that defy belief when it comes to game design. Both Eldar rangers and Guardians have absurdly high damage to infantry_med for their cost.
Combine this with the fact that both of these units come out at the start of the game, and have a couple of easilly obtainable upgrades, they can easilly kill enemy capping units, gain tier 1 dominance and then the Eldar simply move on to a fast tech followed by WS spam for a finishing blow.
To give an idea of how much damage a guardian causes to infantry med. I shall compare to the humble space marine.
A space marine causes 10.6 DPS to infantry_med. A full squad at 390 Req will therefore cause 84.8 dps to infantry_med.
A guardian squad at 325/15 with a warlock causes 102.7dps to infantry_med.
Rangers maxed out at 320/40 do 66.4dps to infantry_Med, the same as 6 space marines, at a marginally increased cost.
In fact, guardians outperform both Sniper Rifles and CSM heavy bolters for cost when killing infantry_med.
(a CSM heavy bolter does 28.0 dps to infantry_med at 90/15, a sniper rifle does 33.2dps to infantry_med at 85/20 and will only do 23.3dps against Guardians since excess damage is lost over the 210hp of a Guardian)
Add in fleet of foot.
Guardians and Ranger's aren't exactly fragile either. Guardians have about 10% more health/cost compared to marine scouts. Rangers have a superior armor type that means that they are almost as resiliant as marines against most space marine weapons, with the exception of scout sniper rifles.
The overall effect of this high infantry_med damage + speed + durability is game breaking. Guardians and rangers can earilly gun down cappers, and Guardians having a detector as well can make a serious mess of stealth suits.
Imperial guardsmen are cut down easilly, combined with other Eldar abilities (FS storm, mind war) making this a hell match for the Imperial guard. What's more, Rangers scale massively and instapwn morale, plus infiltrate, plus fleet of foot.
It's not as if Eldar don't possess other weapons in tier 1 that can't handle infantry_med either.
My suggestion: Nerf the infantry_med damage of both of these units to bring them in-line with their damage to heavy_med and making them similar to other tier 0 / tier 1 squads in DPS (bringing them to around 70% of the DPS they currently put out)
Afterwards, bringing scout sniper rifle DPS down against infantry but reducing power cost to about 10 should balance this all out.