1 – Too much focus put on the breeds going into a crossbreed and not enough focus on the individuals.
2 – Expecting extreme breed features, like the height of a Great Dane or the thick bone of a Neapolitan Mastiff, to stay in a crossbreed. These breeds are inbred for a reason: tandem repeat. Without tandem repeat the frequency those extreme traits are expressed reduce considerably, and eventually disappear. Your F1 crosses my give a 50/50 chance of those traits, but in your F2 crosses that frequency will drop considerably.