There is a power gap between factions, absolutely, and Eldar as the tippy-top top tier will shit all over CSM who are the bottomy bot bot tier (purely on a 'the Codexes themselves fight each other when the humans aren't around'), but the difference between the 'power tiers' is way lower than people who claim they're on the bottom tier like to claim. Orks and Imperial Guard and Dark Eldar are worse than Eldar and Space Marines and Necrons, yes, we know. But are Orks/Guard/DE fucking awful and they totally suck and I can't win any games ever and Games Workshop are all evil bastards and I'm retiring the army to my shelf because I can't possibly win with them ever again? Oh, fuck off and stop being such a princess. Go and play Battle Company or Eldar or Canoptek Harvest spam and you'll still lose like an ass because you get beat by better players who've stuck with an army and learned how to use it properly, not sat on the internet bitching about how terrible the army is.