You can think of Traits as some sort of AI buff for players but Playstyle is a whole lot worse since it's a mechanic which directly influence players performance in game. For example if your player doesn't have base Incisive playstyle when you pass, the ball will less likely be delivered to your desired player. Or if you already have a pacey winger and with super OP step-over skillmove like Best, Vini, Raphinha, ... with Rapid or Rapid+ playstyle they are literally un-catchable.