I honestly think that there are good and bad in each bread. But that it really depends on the horse and who's training them. I have had several different breeds. 2 tbreds, both not hot, one was off the track the other was a rescue. I have an Appy, who is the bestest horse ever, qtr horses, one is missing a screw and is conformationaly defunct, POA's, Welsh, and a new adition a halfflinger. I have seen it all, and I don't think it can be breed specific. But there will always be people who like to sterotype.
I have also seen a person who gets horse after horse, different breeds, and they all end up acting the same, wonder why? G could it be the rider???
Anyway that's my 2 cents.