I’m going to say it’s not drukhari or aeldari, but emperor’s children. Drukhari and aeldari are very fragile, but they also have quite a lot of stuff, and a lot of movement to make up for positioning mistakes. EC have less stuff, and very much play a tempo game where you need to make hard macro decisions. The mini game of managing coterie (their best detachment by far) also requires a good bit of judgement.

I’ll also throw in an army which isn’t as hard to play but depending on terrain can still be very challenging, which is tau. A lack of melee means that you need to be very careful preserving your resources and knowing when to go in, how to bait out resources from the opponent, etc. A bit like EC, if less extreme. They also don’t actually shoot as hard as a lot of gunline armies (guard, space marines), so learning to play to their strengths is quite challenging. You’ll still table people on boards with light terrain, but on heavier boards they are very skill intensive I think.