I believe uke is always right. A shomenuchi may come out as yukomenuchi or something like that but nage then has a wonderful opportunity to train his/her ability to adapt to the situation. He may then end up doing another technique but I don't see a problem with that. If sensei sees the situation from beginning to the end he might correct uke or whatever.
IMO uke does as best he/she can - that's uke's responsibility (even if it is done deliberately). How nage responds is nage's responsibility. What sensei does about it is sensei's responsibility. You're bound to train in Aikido long enough to learn all the techniques so an uke who plays the game somewhat differently from time to time is OK. Whatever he/she understands under a certain technique, be it right or wrong, is still Aikido. His/her own version of it. Like I have my own and you have your own.
In our dojo we don't concentrate in real-life Aikido or how it can be used in situations on the street. We see it more as an instrument through which we learn mostly about ourselves and others. We all have problems and we try to solve them in training. If someone in a role of uke does his shomenuchi with a straight arm or too lightly or too fast or whatever is not my responsibility and who am I to judge him/her because of it. If he/she has less experience in training I may advise him/her with a word or two but then let them do as best they know how.
I agree it is sometimes difficult to deal with an attack that is not sincere so to say, but I also know it is difficult for some to attack sincerely. I had this problem when I began training and sometimes it still shows. Resenting violence as I do, it seemed contrary to my beliefs for a while to go try and cut somebody in half with my hand. Now that I got a better idea of what attacks are useful in Aikido training, I lost most of that feeling. Working on it still though.
That's what I mean when I say we all have problems with ourselves. Let's concentrate on resolving those rather than the ones in our uke.