The advantages of a scrimmage is you get to see players in action against similar talent, assuming you find the right team and the right situation in which the other side is willing to field a somewhat standard u21 setup.
The disadvantages of a scrimmage is that most people will run 1-off strategies into one another, and the end result does not necessarily benefit you as a coach because you do not effectively know if your team defensively is good against an LI or LP when you faced a princeton or outside isolation.
While i am willing to take on a scrimmage against the right team, i've not once really been offered a scrim worth taking in the 6 seasons now that i've been in charge of a u21 team. I'd have taken one as the US manager when Cletus was the canadian u21 manager if we could mutually agree to run something we could both benefit from, and i would equally be interested with the US u21 team in the same format now. Furthermore, any other u21 team that is at or better than the Canadian u21 in terms of talent that is willing to negotiate a specific playtsyle i would be open to.