Re: Difficult uke or bad technique?
The short answer is "both".
Good technique will work against someone locking you out and applying yonkyo with morotedori. My sensei used to do it to me all the time, and working out how to deal with it has helped my technique a lot.
Having said that, when my sensei did this to me, it wasn't in order to make me screw up, it was to help me to learn how to move correctly, and he helped me to do this. If this uke isn't doing this, or can't do this, then it really isn't helpful.
What I would do when training with him is to grab a senpai, and get them to work with you to show you how to deal with this kind of attack. Once you learn, it is really quite manageable.
FWIW, the kind of ukemi that really bugs me is where they grab you lightly, and the minute you move an eyelash, they let go and jump away from you as if you have have punched them. Those people are really hard to work with.