My Scalambrieri played 18 matches in U21 and is 6 in experience.
My Ezell never vested US jersey and is 6 in experience (same age).
Rules say so, right, but impact (if any) is definitely minimal. And even you have some decimals in experience, if your player is kept in bad shape, that will overcompensate the advantage.
About trainers, probably you misunderstood me. A L4 trainer can bring a player in Italy U21 (rather competitive), we usually do so. A L6 provides a major boost under this perspective (I scout a couple of them).
What I meant (and we eventually agree on this point) is that the manager makes the difference. A L6 trainer used for a double spot training is simply crazy.