There is favouritism in UEFA and it's match officials reflect that, it's probably about how much cash each team going through could generate for their coffers.
As others have said it's not that the decision was wrong it's because everybody knows(which is damning of UEFA) that a tight call especially when it really matters(when it has the potential to change a tie) will not be given against a favoured club. A last minute Penalty against a Real, Barca, Bayern side that decides the tie? No chance unless it's a leg breaker and even then they might not give it.
We saw that numerous times against Liverpool, the tight calls all over the pitch, obviously there's the big game changing calls when it matters like Salah offside, Sane onside, Sterling fouled-the player handles it all in the penalty area but not one official saw it? Game changers.
More than that though I noticed niggly fouls they were doing on us in that first half for example barging our players to the deck from behind to win the ball back and keep their momentum going were all being waved on, with our players giving the ref a look of "what the f***?" In the second half he couldn't stop blowing his whistle when our players were just outmuscling them shoulder to shoulder, they would hit the deck looking assured that the ref would blow his whistle, affecting any momentum as soon as it started to build(which we struggled to do from the minute they went 2-0 up). It's subtle but a ref can influence a games momentum which can influence a result we had both that and the tight game changing calls in the first leg going against us, meaning we'd have to have been exceptional to get through against those odds.
Some call that "game management", "buying a freekick", "taking the pressure off for his team" and so on but it can't be called any of that... what it should be called the way it pans out in many of the crunch ties is "using the ref to your advantage" and when only one team is allowed to do it, it can become quite the advantage indeed.