The man I worked for years ago bred from a warmblood mare that was the easiest and sweetest horse imaginable. When she had a year off after several foals I tacked her up and got on her and she didn't put a foot wrong.
Her offspring were also really good, one went on to be Reserve Champion Police Horse of the Year - he only lost out because he gave the camel a 'funny look' rather than ignore it
Our WB X TB here is the easiest of horses to handle and very well mannered to ride, a real sweetie with loads of character yet we were told that in the short time she was at one of the Equestrian Colleges the staff and students really disliked her and found her difficult
If you keep any horse stabled a lot of the time and have it in super fit condition then it will be get 'full of itself' but that can apply to any breed you treat that way