I can take this one on. I'm not a goalkeeping expert by any means but I've played the position all my life, played at a decent level of amateur football and just always been more interested in keepers making great saves than any other aspect of football. Goalkeepers can be weird like that.
My issue with onana is that, at times, he looks like a player that isn't a goalkeeper reluctantly being made to do nets (right down to wanting to make a big show of his ball playing when he does get it to feet). His technique on the basics of goalkeepering really is that poor. I've been banging on here about how so many of his cock-ups are just so unorthodox. It's not just a simple handling error or momentary lapse in concentration conceding silly goals. Often it's this totally unorthodox way he decides to do nets.
I've never seen a keeper so intent on guessing which way a shot is going to be struck and ending up completely wrong. It just looks ridiculous with him falling over before shots are even struck or leaving huge, gaping parts of the net open.
I am yet to see him make a save that has truly surprised me, you know where you think 'how has he saved that?' - he just doesn't make those sort of saves. He just leaves you thankful when he manages to field the simplest of efforts because you are so used to him chucking them in.
Then on to the saves he actually does make; for a keeper so fond of parrying the ball he seems to lack the ability to direct where those parries go. When he does make saves it comes off him at these odd angles, straight up in the air, angled back to the danger area etc. It's just a lottery where it ends up. DDG liked to save shots with his feet more than any keeper I've seen and Onana is the same only with his wrists.
It all comes back to the poor technique, the second FK against Galatasary is the perfect example and the one i will keep bringing up. If you watch his hands closely he has them in a fist the whole time, like his number 1 priority is he's going to slap this ball away somewhere. Then when it ends up as a fairly tame effort coming straight through the crowd at him he's so committed to the slap his still goes for it and manages to do some absurd backwards scoop in to the net.
He is not it and the sooner he can be replaced with a semi competent keeper the better it will be for the club.
I'm an ex keeper too and I dont think he gets to play for for 4 massive clubs with fundamental falls in his technique as you described because that amounts to conceding goals more so than any other position.
His style like many non-European goalkeepers is different, not as pleasing on the eye. Brazil have two of the best now, but others before them very similar to him stylistically. I think non-European keepers especially the African keepers without generalising, are more explosive in how they keep the ball out the net.
I think whats going on in front of you has a massive impact on your decision making. A steady back four, rather than second guessing what players are doing, you know how the defence reacts to different situations.
I think when you arrive at PL, like every other position but arguable even more as a keeper, he wouldnt have played in a league like this where you are either constant under attack or your watch a team low block then quickly counter attack. We have seen him take up some strange positions when teams counter attack. Certainly wouldnt have faced this many crosses.
More than anything else, I think hes trying too hard to prove he can fill the gloves of DDG. Im sure no one would have thought the skinny white lad from Spain would end up being so great. Onana needs to be given more of a chance than half of a turbulent season of uncertainty in general.
Goalkeepering in this league takes a while to settle whilst you are coming to terms with it. We have a lot of new keepers who are making lots of mistakes. What I like about him is hes mentally strong and doesnt come across like the clubs to big for him.
Patience is needed even if aesthetically hes not as pleasing on the eye as DDG. He doing something right to have the second highest save %, and I think some really top saves are going under the radar. The save against the Villa at 2-1, where his weight was on the other foot, was outstanding. There was a save from a point blank header as well but I cant remember the game?