Sejuani never looked too shabby in my opinion, but then again I've never played her. The idea of a tanky character with slows and no damage isn't unheard of. If anything, she's a pretty decent asset to the team if she can peel people off of the AD Carry in team fights. Sure her clear time isn't so great, but you can't have a champion that's perfect in every detail or otherwise it wouldn't be balanced.
Also, Jayce is far from broken.
I can't think of any other tank like that that doesn't have harder cc. No damage + little hard cc = pretty much useless.