It's highly breed-dependent. Some crosses are disasters, I'll give you that. But if you get a good mix, it's because the outcome takes on a blend of characteristics that work, while knowing the possible negatives and addressing them.
A Jack Russell cross will always be a disaster.
Mixed breeds allow you to take on a greater number of positive characteristics than pure breeds can offer. Just think of the hypoallergenic qualities of a poodle cross.
Our family growing up had a cockapoo, which I declare is a good mix for a family dog for a number of reasons. Pure-bred cocker spaniel or poodle? Not as much.