It's hard to generalise with breeds but with what I've seen of thoroughbreds I really don't like them. Sure, they're beautiful and alot of the have potential to do pretty much anything from dressage, to jumping and hacking. But really, the beautiful, filled out and well behaved ones you see in the show ring or competing have taken alot of work. I'm talking literal blood, sweat and tears.
My ott has taken 8 months to fully retrain, feed up and have him in show condition. So much work has been put into him. I've cried alot, been though alot of pain, gotten my hands and fingers torn and swollen, but havent fallen off, thank god. But it really has been worth it. He's never been unplaced, everybody loves him and he's just a pleasure to own now.
But I guess it depends on the individual horse but alot are like this.
Keeping in mind that the above was stated for ones that ahve raced a fair bit. The less they raced the easier it will usually be. Although my friend has an unraced thoroughbred and she's still stupid at her first couple of outings (unlike mine, who raced for 7 years and was an angel at his first outing) and is a b**** to ride.