But M20 mines!
Sometimes I live and die by these mines!
The instant critical repair ability is something I don't use very often. I think on it's own, it is OP. However, in my experience there is rarely a time where I have a critical and have enough time to use this ability. Axis AT is pretty potent, so if there is AT around and I get a critical, the vehicle is lost. Using the ability in combat results in loss of the crew and an abandoned vehicle. Typically if I have time to use this ability, I almost always have time to repair it all the way. Also, almost every axis opponent I play against does not place vehicle mines, so criticals don't happen often. It's usually my misjudgement of faust range. Opponents will usually faust, just because they can, and typically can't finish the job (no followup attack). So with all that considered, not only do I think the ability is not OP, I don't even really think it's even relevant to balance. This is just my experience though. I don't play a lot of 1v1, so if it's being used in 1v1 or even 2v2 I wouldn't know.