Repair tool ribbons - what a bitch of a challenge. Takes forever to even get one ribbon, and you need 50 to complete engineer expert. I've not had ribbons despite nearly repairing from dead two vehicles in one game.
Also I don't think repairs count if the vehicle started life as an enemy vehicle. I hijacked a tank yesterday, needed a big repair, got hit a few more times, lots of repairs, nothing.
If I play for it I can usually get 1 ribbon in a full game of conquest, I think my record is two ribbons. Given how easy medikit and ammo ribbons are to get for the equivalent challenges i think DICE didn't really think this one through, unless the unlock is a monster gun.
Once this is done i am never repairing anything again ever.