I'm not sure that tracks. We're talking about robots. Clearly he has significant skills in repairing electronics. It's like expecting a heart surgeon to not be able to treat a gunshot wound. Perceptor was qualified. He was familiar enough with the inner workings of transformers to guide Brawn and Bumblebee inside Megatron's body on a micro level. I mean technically, Ratchet is described as a "medic." Traditionally combat "medics" aren't in the same league as full surgeons, and usually aren't even Doctors. It's entirely possible that Perceptor is actually MORE qualified than Ratchet.