Too much randomness. Shitty drawings, the scheduling in general that may or may not create ridiculous match weeks (think for example having to play Bayern between matches vs Real and Sevilla) for one team and not for others, meaningless matches vs fodder teams in the group stage that basically favours teams with depth rather than excellence due to the resulting fatigue issues.
It's also more of a marketing cup than anything else and completely overvalued due to that fact; in reality, most of the matches fail to live up to the created hype and the team that wins the competition is "the best team of Europe" on paper only.
A domestic double on the other hand means mastering your competition over the course of almost an entire year and several times as well, so it's a way bigger feat in my book.
Other than that, it's a no brainer that Barca is capable of reaching the semis in the CL, maybe more, but it's ludacris to expect (!) to win this thin