From my PoV, Illidan is not the hardest encounter in the game.
Kael'thas Sunstrider was way trickier and had more work behind it from a raid point of view(Before the nerf of p1)
You had like 5-6 tries at him before the raid ended(seeing you raid for 6 hours)
Due to many easy wipes because of small mistakes and/or trash respawns, so i'd say, when you first start taking on Kael as a guild who just got there, you'll struggle more on him then if you had the same situation at Illidan.

(Please comment if you feel like it, but don't bitch as this is from my Point of view) kkthx