Are mixed breeds at the shelter a good first time dog?

If they are, why?? I see lots of people saying, go to your shelter and pick out a mixed breed. They are great for first time owners. I think why??? what if there is a chow chow rottweiler mix there?? that wouldn't be a good first time dog!! so can someone explain to me why mixed breeds at the shelter make good first dogs??? Thanks lots!:-)