As a PSG supporter, i'm pretty impressed by Emery's results. He never really managed to install his playstyle in Paris and sometimes had a tendancy to overstress players on the bench. I think he's the kind of coach that thrives when he doesn't need to prove his worth to his players and have a hard time managing big egoes. Arsenal is a perfect fit for him, there's everything to rebuild there and he knows how to do that. Arsenal is a big historic name and players don't seem to think they're above the institution so the coach doesn't need remind them of this fact constantly. Which is good for Emery because from what we've heard, he caved easily when a couple of players came to his office telling him that it wasn't the way the team was supposed to play.
He made some very weird choices in Paris though, leaving Thiago Silva on the bench and playing Lo Celso as a DM against Real Madrid for exemple. To this day i can't understand what the hell he had in mind. Obviously, it didn't work.