Liu Kang seems to be quite troublesome. You have to hold his pressure and he can easily punish a whiffed reflect with a MB flying kick. He breaks your armor like a toothpick as well.
This is still debateable but I feel Mystic Ermac causes problems as well. Particularly because he forces you to come in. I'm also pretty sure Tele -toss doesn't punish it force push either.
Dont know about the dlc characters yet but Cyber-Sub can be a nuisance.
Then there's Shinnok..
@Pig Of The Hut and tell you all about that S.O.B.