Say you pile 6 nets on a player (3 primary, 1 stamina, 1 field, 1 w/k) he won't be able to play much at all.
2 nets on 3 players (1 primary 1 stamina, changing to a fielding when his stamina is high enough ) The 3 players can play the odd game and be rotated. After 2 seasons their stamina will be such as they will be useful players.
U-19 players can be great fun, but it simply isn't as effective for improving your side.
Although if you have a very well balanced team, it could be worth piling 5 nets on two players, who after 5 seasons or so will be absolute stars. Which will be slightly more beneficial than training 5 players to around the current skill level of your side.