I'll just chime in here and remind every one that, averagely speaking, advantage translates to a +5 bonus, even if it doesn't stack.
Basically. That’s why GWM is utterly OP right now. You can backstab with advantage that negates he penalty of the power attack portion of GWM and now you do +10 damage per hit without any penalties. It’s one of the main reasons why people are soloing this game.