He is one of the most successful club managers of all time, this is without doubt.
He could win his 2nd treble this season, no other manager has done that. 6 trophies in a season at Barca, again not done before.
He has dominated 3 top leagues to a level few managers in history have ever done (Carlo for example has never dominated a single league). His Barca is one of the most dominant domestic Spanish team of all time, likewise his Bayern, and likewise his City.
As to "If he is so good why has he struggled to win the Champions League in 12 years"?
The Great Sir Alex only won 2 CLs in 20 years. Is he still not great? Mourinho, 2 in 20. Is he not great. Pep is on 2 in 14, if he wins this year he'll be on 3 in 15, which is more than Sir Alex and Mou in far less years.
Easily the most tactically influential Manager of the last 30 years, his imprint on the game is massive. Who popularised the false 9 craze? Who turned attacking midfielders into proper centre midfielders in the hard fought Prem when most said it wouldn't work (De Bruyne, D Silva, B Silva)? Turning a mediocre C midfielder into a makeshift left back and getting 100 league points? (Delph) Winning back to back Prems without a striker? I remember watching a Barca game where he had just 2 guys at the back (Masche and Puyol) and 8 guys in midfield (2,8,0). The guy is so good he does what he wants, I mean we saw him rock up at the Emirates with B Silva at left back, toying with Rico Lewis as an inverted full back next to Rodri, with Cancelo, Walker, dias and Laporte on the bench.
He is already widely regarded as one of football managers GOATs. By the time he finishes managing, he will be the undisputed greatest.
He is without doubt the most coveted football club manager in history, and with good reason.