I used to instruct regularly in owners aircraft. You get to fly some interesting types and often fear for your existance. Especially so when operating farmer joes C172 station hack.
By far the most awful aeroplane I flew was a socata rallaye. God awful, gutless, slow, hot, worse than a tail wheel to taxi. The slots would open whenever you hit turbulence and the wing would drop. This made nav training nausiating.
Yuck.