It's very, very hard to tell until they play at least a handfufull matches for the first time, so I wouldn't be quick to make a sweeping judgement on the overall academy producing multiple players from the current batch. Then the smoke clears a bit and you get a reasonable idea of where their career could be. Not exactly an idea of their upper limit, but you can tell if we're talking about United quality.
For example with Mainoo it's fairly clear he should be in a United squad for years to come, unless something goes drastically wrong with injury or his focus. Don't know his end potential but being conservative it seems clear he should be around the squad.
But with Kambwala he's coming in and played a game of football. Can't really tell anything from it, anyone can have a good game. Same for the rest of the youth, many have looked world beaters at that level.
But to me if you can produce a player for your squad once a year, that would be an amazing result. It's not easy even to even produce at that output.