Mutts are not always healthier. They can have just as many if not more health issues. You could get the worst of both breeds instead of the best or you could get a total Heinz 57 mutt that has a lot of issues. You could also get lucky and get a super healthy one but not always.
I love mutts. My brother and SIL have 3, I have my own, neighbors have one. Love them all. Mutts are fantastic. So this is nothing against them. But they are NOT always healthier and definitely not better behaved. 
 
They CAN be but that’s not a guarantee or a fact.
There’s lots of untrained, horribly behaved mutts in shelters. Purebreds too. There’s also well trained dogs. Both purebred and mutt. ANY dog can be well behaved. It’s a matter of TRAINING and how much time someone puts in. Not the actual dog. And a well bred purebred will be very well behaved if the effort is put in to train them. If not, maybe not but the temperament will be there.
Every mutt that I know has their issues along with their fantastic qualities. We still love them and would get more but it doesn’t do them any good to market them as these perfect dogs when they’re not. What if somebody went out and got one expecting perfection and then it had health and behavior problems and they returned it? It’s better to be honest about ALL potential issues upfront.