As a "newbie" to the art and after reading this thread there seems to be some conflicts of opinions (what a forums for but hey).
Surely when practising either as tori ar uke you need to know the technique is valid and works. Personally i prefer to be hit at full speed with most techniques as actually having the technique applied correctly as uke provides a very valuable learning experience, i really don't think itsa all about ukemi.
As for the instructor bleating well surely he should have asked the relevant questions, of why the technique wasn't working and corrected it.
I do have some personal experience of this, although i wasn't admonished for it. Sankyo and Nikkyo just do not work on me at all and this was discovered when i was uke for sensai too. Rather than get flustered or have a go he simply modified the technique to make it work, then explained that in the event of it happening this is what you do.
I suppose i could have been dishonest and taken a knee for it but what was the point, due to being honest i learned a variation that may well serve me if i ever meet another freak like me
Unfortunately the downside of this is i now have a line of dan grades every time i train trying to apply either nikkyo or sankyo.