Modern football requires a goalkeeper who can build attacks from the ground with his accurate passing. But what happened to Machester United is that they neglected the things that a goalkeeper should have, namely good reflexes and also a good reading of the direction of the ball, and that is now not shown by Onana.
I would largely agree and say the same thing about Manchester United, which is that they have made a mistake in discarding David de Gea, although his weakness is not having good passing, in my opinion it is a weakness that can be corrected.
It's true that modern football demands a more well-rounded goalkeeper, one who can not only make spectacular saves but also contribute to distribute the ball effectively. While David de Gea is a world-class shot-stopper, his issue is not only passing, but also confident with the ball on the feet, decision making on where and when to pass the ball. He is also kind of GK who stays on the goal line, rarely claims the crosses and not good as sweeper.
On the other hand, Andre Onana's passing ability and his confidence with the ball in the feet is undeniable. He is comfortable on the ball and has a good range of passing, which allows him to initiate attacks from the back. However, as you mentioned, his shot-stopping abilities have been questioned. He has made some high-profile errors in recent years, which have raised doubts about his suitability for the defense structure of Manchester United.