I'm thinking this should temporarily incapacitate everyone who donates or set them back somehow for a little while. That would be the drawback; it leaves everyone vulnerable for a bit.
Perhaps they actually transfer a significant portion of their powerlevel (and the additional ki it allows them to hold) to the recipient until he/she makes an attack? This would leave them ridiculously weak (as in the movie) but still able to fly, use some techniques, etc. until the person they donated to makes an attack, then they get their powerlevel back. They'd have to refill their ki pool though, because transferring it "exhausted" them.
Hmmm, I'm not sure whether this should come from ki or pl or both. Admittedly, I like the direction you guys are taking it more than my original one. If it was just ki to boost powerlevel, would it be enough of a drawback for everyone to just have lost some of their ki pool? Should they be drawn into the powerstruggle (i.e. unable to move while it's going on)? Are team members donating ki, powerlevel, or both? Haha, should this allow a kind of "team powerstruggle"?!?!