After months of struggling and getting mostly losses with my first army, DE (4 month loosing streak was the longest) I'm finding eldar with DE allies to be easy. What happens is in round one most of my DE are one shotted. Leaving the eldar left to close in for the win. I hate the fact that my people are the focus fire targets before the Eldar are.
Last i got was 3 wins with DE only then I have had 5 wins with Eldar and dark eldar allies so far against competitive players. After struggling with this, from what I'm told 5th edition rulebook codex, playing another army is really simple :-) . It brings a new perspective to how to play and has given me better ideas of how to manage a DE only army.
It feels too powerful because of the fact the wave serpents get glanced to death then guys come out shoot and charge. I am 6 and 2 so far against competitive players. If I go full eldar they won't blow up at the start.
Anyway, my point is, I don't think other players know how easy they got it when playing as another army. I will keep trying DE allies/only lists and some Eldar only lists but I think we got the short end of the stick and one of the most challenging armies there is still.
My to cents/experience.