Joao Felix isn't really a terrible buy per say but what I understand about people's dismay about his transfer fee to Chelsea is due to his poor performance while he was on loan in Chelsea before later going to play for Barcelona. Even in the just concluded Euros there was no much of an improvement either. However, coming back to Chelsea he now has the opportunity to work on his lapses improve his standard and be more creative than the last time he was there.
Joao Felix is a "moment of Magic" kind of player. Top players try to make something happen five times out of ten times they get the ball in a good area, elite players like Neymar and De Bruyne try to make something happen seven times out of ten times they receive the ball in a good area. A Player like Messi makes something happen nine times out of ten in a good read. With Joao Felix, he does that at most two times out of ten, and this will have you wondering why he's like that because you see him play you'll see he has all the qualities but he just can't do it consistently on the pitch. He's not the kind of player you can count on in my opinion. He's more of a backup player.
A player that's are inconsistent as him shouldn't be rated that highly. You'll go into a match not knowing what to expect from him. He could go in and be an absolute menace to the opponent or just ball-watch and give away every ball he receives. He's not worth it in my opinion.