Depends on how good you are! It took me a while to get used to it but then it depends on the model too. As a guide I reckon it took me the summer to get it good, but then some days I was awesome and other days I was just awful! I think the hardest thing to get used to is the brain telling the thumb the correct inputs as you rotate. Its easier if you can get the aircraft to stay in the attitute that it will naturally TR in.

As far as the hardest stunt goes I think IMO its the rolling circle or rolling loop. I guess the rolling harrier is quite hard but I've not attempted it yet. I think its because all these require all 3 controls to be moved at the precise time by the precise amount for it to work. And no I can't do a rolling circle either!