[MENTION=11668]khaled_a_d[/MENTION]
I see the point you're trying to make but IMO, separate instances of coaching success/failure do give an idea into the ability of a manager across a multitude of conditions. Otherwise, all we know about the ability of a manager is purely and almost entirely circumstantial.
Your example of Pep kind of buttresses my point. When he left Barcelona in 2012, he had an almost divine standing in the game. But after repeated CL failures at Bayern/City (some his fault, some not), we now put him in the same class as a Klopp for example, and not head and shoulders the best (which he was considered in 2012).
If EV wins a treble/CL with AM, it would actually show that he was carrying the hell out of Barca 17-19. If he fails to win any major trophies with AM (which I think much more likely), it's another piece of evidence that he's just not the manager to put a team over the top.
It's not easy to win thing with AM. Even Cholo wins just 2 Liga so far (1 Copa del rey and 1 Europa). There are two UCL finals as well.