I am using these examples because this is what I'm familiar with.
When you're choosing a barrel racing prospect, who will you choose?
Tres Seis, a racing quarter horse?
Or this warmblood, Negro? (I know nothing about him dressage ladies...So feel free to enlighten me if you want, I just googled and he came up)
Anyone with eyes can see the difference between these two horses. Tres Seis built to sprint, and absolutely throw his hind end in the ground and run around barrels; Negro is not. He's built for good, big, dynamic moves that will earn him dressage points. You tried to run barrels on this horse and you would break him. You would break Tres Seis too if you tried to jump him like a lot of warmbloods do.
Heart can only make up for so many faults...Some horses will try until they kill themselves. Just like some humans. You, as the owner, have to know the horse's limits better than they do and stop them before that happens.