Jotaro Koma is a great prospect for Japan and will be an amazing player like Matsuyama can be currently.
So far, his manager is doing a very good job to make him progress, but first, and to properly evaluate this method, it seems important to me to specify the levels of its trainer, and of its young trainer ( I think he have one!), as well as the levels of its infrastructures like Gym Center and Training Court. It can make a big difference.
Then, it is also important to know that Koma, with his Hall of Fame potential, was drafted last season at 18 with a TSP of 65 (40+25), which is an already exceptional profile. I told you about him Yellow Cake at that time.
At the moment, I remain a little skeptical too, I think it might be a special case in very favorable conditions, but I may change my mind if more examples are added.
In any case, the undeniable advantage of this method is that the player is operational in defense more quickly.
On the contrary, in his defense, and as Orgram pointed out, it only allows 3 players to be trained from the start.