Well they gain the same amount of skill progress as training, the advantages of a friendly is extra booster packs, the disadvantage is more chance of injuries, but if you create a "punchbag" account and set it in a illegal formation and set it on easy tackling then it should be advantageous.
I play 2 friendlies a day against my punchbag teams and 4 friendlies on days when I only have just the 1 competitive game.
*Retired From Top Eleven*