You learn life lessons in any situation you open yourself up to. Life lessons are learned in all sorts of places.
An assistant coach, a head coach, an equipment manager can teach you lessons if you are willing to learn.
But a coach of any kind can also bring you down and/or break you. A coach seems to have more influence over you because they are an older authority figure. That's something a coach of anykind needs to know; while they can build you up and teach you how to be a man, they can also bring you down and break you if they aren't careful.
now to answer the original questions...
Sometimes an assistant coach is more approachable than a head coach. Sometimes the head coach needs someone to step up and pull his weight.
Sometimes an assistant coach is there for right reasons because they sure don't get paid much money.
Sometimes an assistant coach spends much more one on one time with a player or a group of players, therefore could be hugely beneficial or hurtful.
Really, it all comes back to the head coach. I think an assistant coach can pull a program either way depending on the direction and leadership of the head coach.