I'm sorry, but Cron's, CSM and Sm are not difficult to play, i expect while a few of ppl who say they are may be vets who see MEQ killer lists daily and in that case fair enough. But to me the majority will merely be attempting to justify picking an army because they went 'Wow, 3+ save' by claiming that rly its hard tactically to use.
While i play eldar, a fairly tactical list it is by no means the most challaging to me (i can see arguments for it however), eldar do have the advantage that every unit is pretty much stated what it does so do it like tht or die! I voted DE because as someone earlier stated, open topped AV bottle rocket tanks will always be hard to use, combined with troops that fall over in a strong breeze this team rly does require a good idea of how to utilize cover and the battlefeild to work in their favor, hats off to any DE players. Also noted i don't count Inquisition as my above mentioned easier armies, as they cost so blasted much.
P.s. before i get the inevitable hate mail, i've never collected Inquistion so not attempting to justiy it but i did get Space Wolves.