Arsenal, Roma and Juventus are not better than Manchester United. If Manchester United can eliminate Barcelona their most deadly rivals, they can do that to any team. It is that simple. The winning mentality of Manchester United is so high for now. The new coach of the team carries pressure and the players are more United than before.
That's true, but for now Manchester United still has to see other rivals as a strong team like Arsenal's example so they don't take other opponents lightly for them. After all, Arsenal are also not too bad this season and will definitely do better in the Europa League as well as in the Premier League this season.
You are right Manchester United are in one of their best form right now. Despite being in great form, Man U managed to beat Barcelona. Now no one can stop the Manchester United team.
If you say nothing can stop Manchester United now it's because of Manchester United who have been able to beat Barcelona in the playoffs. I think that's too arrogant to say because other teams who are already in the round of 16 now also still have the power to surprise even more with the games they will face this season.