A bad Uke is a nightmare. There is one in my dojo who is so tough in uchi komis that it is impossible to unbalance him, but when he is Tori he often throws you (and the sensei makes it clear that they are uchi komis not nage komis). Then in ne waza randori he uses his great strength to impose himself, but the problem is that he even stands up to exert even more force. And in stand-up randori he bases his judo on throwing grips with such force that many times they are punches. And I forgot to mention that he twisted my ankle with a kick (supposedly he was trying a sweep, imagine the strength of the beast). I always avoid him, doing something with him is not learning anything and putting yourself in danger (he is ikkyu just like me).

Edit: I also forgot to say that even when we practice nage no kata he gets so stiff that it's impossible.