I think Emery will succeed. Arsenal do not have a bad team, they just have poor team cohesion, bad organisation, defending and discipline, all that can be fixed by Emery.
Arsenal's struggles last season were away from home where we conceded too many goals to shit teams. At home, we nearly won every game.
Furthermore, Arsenal needed a dire change in management and internal structure because the Wenger regime had gotten too stagnant and backwards.
For these reasons, as well as a decent transfer window, I am confident.
As for Emery's tenure with PSG, I can't really comment on it. I don't know how well they played or what their weaknesses were. The champions league is really a crapshoot sometimes and anything can happen.