Yes, there are many different levels of resistance, but who is to say what is the "right" way of resisting? Everyone resists differently, and by practicing with many different types of resistance we can learn many aspects of the technique in different ways. Therefore, if we attempt to "teach" tori with our resistance and instructing him on how to go about executing the technique, all we are doing is teaching him how to deal with someone who is exactly like me. (What are the odds of that?) All to many times in my experience, I have "taught" an inexperienced tori how to do the technique properly, practiced it over and over again with them while giving a good amount of resistance, only to watch them get stopped easily but the next uke. Why? Because the next uke's resistance is not the same as mine. So, I say that if you are not the teacher, then you shouldn't be teaching. The uke's "role" is simply to attack the tori. Period. And an attack without any resistance is not an attack.
An important aspect in dealing with attacks is acceptance; accepting that this is what I have to deal with, and accepting my own ability and limitations.
Now, as was mentioned, if go to do an ikkyo or shihonage and someone stops me by pulling down with strength and commitment, and I cannot do the technique, then the real fact of the matter is that I lack in the ability and know-how to deal with this type of resistance. We can't go blaming uke every time we get stopped. It would be better to ask for help, do a little introspection, or keep practicing until we can transform our techniques and ourselves.
I think there are two main types of resistance. The first type is where the person A seeks to do harm to person B and will resist any means of defense. This is the type that we seem to be speaking of, which consists of many levels of strength, speed, direction, force, etc.
The second type is when person A withdraws an attack, or has not even committed to any attack at all and resists person B's technique by attempting to escape, withdraw, becoming immovable, etc. This type of resistance will stop any Aikido technique every time unless person B is forceful and applies the technique with strength. Many times this occurs when a technique is dead, or when uke is hesitant, or when uke is egotistical and sees himself and the defender, the winner in a "you can't get me" mindset. If this type of situation is encountered, then I find it best to become yielding, and give up. My "failure" to execute a technique on someone of this nature actually results in a mutual win, which is the way of Aikido.