I don't it very well ;-) It's very hard, when we buy a horse, we have 30minutes to buy him when we are in a foreign country: we look at him in his stall, then we ask to see him in liberty, and we ride him to see if we like him.
A very beautiful horse can be less beautiful when ridden. We ask the breeder how is the mental of the horse. We have criteria but we can always be disappointed. In our discipline, even if we are disappointed we can find something to highlight the horse.
We don't need a Grand Prix horse, every horse can have his role. We had to bring back 2 or 3 horses because they could have a dangerous behavior with the crowd, it can happen but it's rare.