I agree with all the above and the comments of
@catcherintherye . Unfortunately, though, U23 football is totally inadequate for preparing youngsters for the rigours of 'men's football', whether that's PL or the lower divisions. Only the outstandingly talented can make the leap from youth to PL football, and they are few and far between.
I don't think the people at Everton are daft enough to send a youngster out on loan thinking that it's going to be easy for them and, no matter how they play, they are going to be in the team every week. They are surely looking for clubs playing at a level where they think the players will be challenged but have a chance of succeeding. They got that spot on with Warrington at Tranmere, and hopefully at Fleetwood this year.
As for Onyango, as I've said before I'm not surprised he's lost his place at Burton. His challenge is to adapt to a tough competitive league rather than gliding through U23 games, showing occasional glimpses of ability then disappearing for the rest of the game. I'm sure the coaches who sent him out on loan knew that, just as most of us who've watched him at junior levels would.
It's not the end of the world for a player if they have a bad loan. There are many examples of that, the most notable for us being Gordon at Preston. Players can still learn from that experience - some will, some won't. Others will find their level at a lower league, but the odd one will still have a chance of making it with us.
By the way, I've been following the progress of Thierry Small, now on loan at Port Vale. Not going too well, only started one game when he was substituted after 70 mins, made a few sub appearances but hasn't been in the squad for the last few games (don't know if he's injured).