Mobs on teams (except shulkers) do not act teamed.

When mobs are summoned using the /summon commands on a team they will not act like they are teamed. Eg: attacks players on same team, snow golems attack zombies on same team, etc. To reproduce summon a (hostile) mob on a team and summon something it would attack ( or put yourself) on the same team. Shulkers are fine though. To make it easier to tell which one is one which team effect them with glowing.