If Man City don’t make a CL Final in Pep’s time then you could say he fails. Mancini, Pellegrini both won domestic titles & that’s all Pep has matched. He was brought in to win the CL & he has to.
You have a point, but would you put aside all his success with city simply because of one trophy, if city win the league this season, that'll be three in a row not to talk of if they add the league and fa cup, that'll be total domestic dominance and that's some feat worthy of note as well as something city wants, yeah the previous managers had various levels of success but I don't think any would be able to match peps success when he leaves.
The UCL is indeed a coveted title that city would be craving to win, pep wants it, the board wants it as well as the fans, and i believe city would push with all they have to get it this season, but if they don't and can make up with domestic titles, then it's a successful season I must say and I doubt if he can be called a failure.