I find that most of the people who say "raiding is easier" are the ones who just clear LFR and base their opinion off of that. The accessibly to raiding is easier as in, anyone can get into raiding. But when it comes to hardcore raiding, raiding is just as hard, if not, harder than it was pre wrath.
Pre nerf H 25m Ragnaros, yogg 0 light 25m, and 25m H LK are probably some of those most difficult encounters in WoW history. Harder than any Pre wrath boss imo. (inb4 C'thun, it doesn't count since it was bugged to be un killable and after the bug fix, he was killed within hours)
imo, H Blackfuse, H council(From BoT) and H Dark animus (From ToT) while it was current were some pretty fucking insane fights too along with their endbosses (Sinestra, Lei Shen etc)