My horse spent three years in a TT, ridden by beginners, on a trail string. Why? Because he was hard to handle, young, and he needed to work to earn his keep. The result? A horse with severe bit evasion issues, who hangs his mouth open, and bucks (head-between-the-legs-bronco-style) when he is bossed around or handled to roughly.
Luckily his problems were solved by getting rid of the TT and getting him on something strong and clear yet not harsh. So I tried "dr. Cook" And found that Caleb reacted very badly to the tight noseband. So I put him on an english hackamore he responds very well to it and he never acts up like he used to on the TT.