This is from the rules.
Having your player on a national team will not affect his player ratings or weekly minutes in any way. He will garner experience, but cannot suffer an injury that will carry on after the game.
I am sure I read somewhere (but it may be wrong) that you get the most experience from NT games,then cup games,then playoffs, then regular games and the least from scrimmages. You get nothing from private league games.
this should be an old rule not updated. If you got injured in an NT game, your injury will disappear when game is over. I think that happen till S6/S7. With new rules, no injuries, no experience (or my Scalambrieri with more than one full season in U21 till WC SF should have had much more experience than Lisanti)