The single target tanks aren't too bad. It's the ones with AoE that are hell to deal with.
By definition, tanks will have CC--and lots of it. It's their job to punch through the line and make life hard for other people, after all.
The problem becomes that when you're up against 3 or 4 tanks, they can keep you disabled for a long time.
The main problem is that every char's ult in this game does roughly the same damage. Some is more front load, some more bursty--but ultimately they deal roughly the same damage.
So if multiple tanks can keep you CC'ed for 12 seconds, and their ult still work as well as DPS's ult, then they become overpowered.
I have survived some ult bombs before as Soraka (Wish + summoner heal + lots of MR)--they have a really hard time hurting me after that. Of course by that time it's 1v4, and they eventually took me down. Keyword is eventually--their damage really dropped after that first ult bomb. The problem is that generally, you won't live until then.