I think the only way the parachute could really be broken when added would be this:
1. The parachute and be turned on and off instantly and repeatedly as well as being able to be used at any height.
2. The parachute does as Buu suggested and resists minicrits which effectively gets rid of two (Direct Hit and Reserve Shooter) of the solid counters to this weapon. Because lets be honest, there's not too much that can effectively counter such a weapon, it makes you harder to hit, making it difficult for any projectile firing class to hit you and that leaves the squishy classes, close range classes, and the Heavy which would probably be your only real issue, a good sniper could put a damper on things but a good sniper counters near everything :P
3. The parachute makes you descend far too slowly, keeping the Soldier airborne for a good amount of time, making it the perfect 'corner hugging' weapon.
That's about all I can think of that don't include game breaking ridiculous stats.