I think it is fine- the crew must jump out to perform the repair. That leaves them not only vulnerable to fire when repair, but the vehicle and the crew is vulnerable in general. If they can manage the repair in time and drive away without sustaining further damage, good for them.
That's my main problem with it, crit repair is instant. Case in point, the other day my Easy Eight got Engine Damage from a Panzerfaust. I was about 20 meters from a Pak 40, very low health, but a good position. I managed to wheel myself behind a shed, forcing the Pak to move to in attempt to hunt me down.
I popped out, just as an MG 42 was passing by. He started to set up, but since set up takes 3 seconds, and Crit Repair is instant, I was able to fix my engine and get back inside because the MG was ready to fire, let alone before the Pak made it around the side of the shed.
It's complete bullshit, if they're gonna have Crit Repair, make it 30 munitions with a duration of 25 seconds. It should let you back into the fight quicker, not let you escape certain death because you have a sack of magic fairy dust that instantly fixes anything.