Jumping on playing vs vs a bunch of people who aren't very good isn't going to give you any indication of how difficult it can be to defend online. You need to play good players. Defending vs the AI is mind numbing and boring even on superstar. The AI is trash and hardly pressures at all, just runs away from players and makes stupid play constantly. Being able to defend that is far too easy.
Playing EASHL vs 5 real good players and defense is a very difficult job. Not that it should be easy at all, I really do enjoy the challenge of it but where it gets to be less fun is when you do everything right and get screwed by the game. Pokes that go nowhere, forwards that can't lose the puck no matter what you do, L2 ballerinas.... all this stuff you don't see until you play better players.
Playing D in this game at a high level is not easy and as everything else, some make it look that way but it isn't easy.