There are 2 downsides to Boogie:
1. His temperament. I will not rule him out for this. Draymond Green has many similar problems to Cousins, but Green is in a better situation. Who knows what he would be like on a bad team. Still, it is very risky trying to build a winner with a guy who can turn on anyone when he feels satisfied.
2. Moving Horford to PF greatly decreases Horford's value. I wouldn't worry to much about #2, but it is pretty clearly established that Horford is a much better asset at C.