At the start of their formation it is easy, you always start off with 1v1 SF/PF. Therefore you set them as subs in a league game, and starters in the Cup / scrimmage. It lasts 1.5 season.
After that it becomes more complicated. They require monopost PG training for OD and passing, you have to couple them with a good starter who still needs training which you then field in your home game (or at least the very game you expect to win for a particular week) and the younglings in the other games. It is tricky, but doable, especially if you start off later in the season when you have already taken some advance.
But you're right, training has a high cost when it comes to performance. This season I couldn't properly use Beerten and Barzilay because they're fielded at positions which aren't optimal for them. No choice, if you want a steady stream of good players you simply can't stop training them, or you'll have to leave BBBL.
Many teams do so, and get down one or even 2 league levels in order to rebuild, I don't like the idea of it, but you can't argue with the results, judging by the successes of AmenRa, Mizu, and as we'll later experience, Joe.