1. Uke is always right.
2. in other cases look at the point number 1
Of course some people will always find a million cheap excuses to justify their poor technique, don't be misled.
Your partner is very right to introduce ‘difficult' attacks; his job is to guide you out of your comfort zone. This is only way only changes can be done in your body, because such situation force you to find new solutions, and consequently it means a jump to higher level of understanding aikido.
I don't think this is necessarily accurate. There's a great piece somewhere by George Ledyard (I'll see if I can find it) in which he writes that he can throw a yokomenuchi that even O-Sensei couldn't shihonage. Anyone can, if they know what technique is coming and set out to thwart it. That's not being realistic or difficult; that's creating a situation built on the premise that the attacker can read the defender's mind and the defender can't alter his course of action. Obviously, there's not much use to training for that scenario.
I myself used to work with an uke who, knowing that a kotegaeshi was coming, would stop his tsuki short and plant like a statue to deprive me of any momentum to work with. He would smirk at this as if he had beaten me, but the truth is that he had just performed an incomplete attack that (a) gave me no reason to defend myself and (b) left him open to all manner of attacks.
There is absolutely such a thing as a bad uke. Just like nage, uke has a job to do, and if he does it wrong, there can be no technique.