Heh !
This is getting interesting > Better arguments this time around instead of dismissing it outright without suggestions or a solid example that would break the game play.
Anyway..
Doctrine based units or abilities cannot be used. So If you are going Armor and your mate goes infantry you can’t build howitzers or call in off map artillery. No Hummels, Perishings, Calliopes,Tigers, Kts , Hetzers, Jpanthers, Rangers, Airborne, Storm troopers, Commandoes etc. That would definitely kill the game play.
Pe can lend his Panther battle group to his Wher/PE Partner but if the partner is pop capped s/he can’t use them. And it woud still cost him 1000 MP. Maybe more. .say 10% extra ? Not a usual scenario to have that much mp floating around I imagine. Hummel is a doctrine unlocked unit. So you can’t lend it nor borrow.
No base structures either. So you cannot lend your Brit HQ to your partner
I can see this being a wild card in late game desperate scenarios.