I don't think cyborgs are underpowered at all. For one thing, they can get the nanoprobes upgrade which helps them heal ridiculously fast. For another, they get two moves and ignore terrain, which in and of itself is a huge tactical advantage for an army you want to keep on the move. They can intercept air units better than mechanized infantry so they also make excellent defenders. Finally, they are dirt cheap. With a good military industrial complex, I can usually have most of my cities producing one cyborg per turn. There is very little than can stop a constant flood of cyborgs. I've had games with close to a hundred of them. They are a great land unit.