Re: "Uke from hell"
What Mary said, too. Very to-the-heart-of-the-matter.
So what is it that makes a good uke? I have, and still do, find this question to be a big fat riddle. I've had a recent break from aikido, and this was partly due to the uke-nage training model, which I find can be very frustrating.
So... 3 suggestions for good uke-ness:
Uke allowing nage to request variations in attack strength/intensity (but not integrity). It's boring not being able to do anything, and boring practices end up becoming no practices. Sometimes, as nage, I want to test myself against a strong grip, and sometimes I want to work on something other than the beginning of a technique. If I'm failing to manage to get things moving even 1 time in 10, then maybe uke needs to cut me some slack so that we can both do some aikido.
Uke seeking to make a connection and then seeking to follow it *whilst keeping him/herself safe* (Very important - I once knew an uke so bad that I nicknamed him [to myself] ‘Atemi ___'. Not because he used atemi, but because he taught me so much about when in aikido you have opportunity to strike. The waza just don't work correctly if uke's being a numpty.) Personally, I have often been guilty of not sufficiently connecting and trying control nage's centre, thereby making it more difficult for nage to practice (perhaps the opposite of an overly strong grip).
Uke - knowing that we all ‘dig in' sometimes, often without realising it - being OK with nage changing the waza as he/she sees fit. Egos again, I suppose.