I think he's a strange one. I also think he's a slight proof that stats can be very misleading. He is ranked third ? for shot stopping in the league. However, watching all of the games and paying extra attention to goalies, he's surely a below standard shot stopper. That is not a question, btw. Schmeichel commented on his technique saving, or rather being unable to save, low shots. His reach also doesn't seem anything special. Fairly ofte he gets his fingers to a ball, or even his hand, but the ball still escapes him. It's happened a few times. Then you have a fair few shots that are debated could he, should he have saved it. You don't get that often with goalies like Allisson, prime DDG etc. When they are beaten, most goalies would be beaten on a general basis. With Onana, there are so many shots that aren't mistakes per se, but they are saveable. That's a huge concern - if he saves those, we clearly concede less goals and thus we are in a better position to increase our point tally.
Then you have his commanding of his box, or lack of. I can't see much of an upgrade here. He also chooses to punch the ball where others such as Vicario, Pope and Allisson would catch the ball. We are terrible defensively on crosses, and he's not progressed our play here much, if any.
His sweeping has improved our high line to some extent, but he's still far from the best such as Ederson, Allisson and Vicario.
He is extremely press resistant, though - maybe even more so than Ederson, which is saying something. His short passing is also very crisp. However, I don't think he's quite Ederson level, especially not with his long range passing, which is too often over hit. He strikes the ball extremely well, so when the direction is spot on, he can deliver some really terrific through balls - but even DDG did that from time to time. He needs to be more consistent in his long range deliveries. He also takes too long on the ball for my liking, which is detrimental to our style of play and progressing the ball.
Overall, I would characterise his overall contribution thus far as below average. For the price payed, it just hasn't been good enough. I think we need to give him one more season, but he needs to improve this second half with Casemiro and Martinez coming back from injuries. With them and Shaw on lb, he has very few excuses in terms of relationship with the defense and playing out from the back.