They're very hard to damage, and can be much stronger and faster than human bodies. (It depends if you get a military model or not.) They don't need to eat much and use less oxygen, though they still need some food and air, because part of the brain is still biological. They don't age and can look however the character wants (for a price.)

There's that learning curve, and they're heavier than a biological body--not ridiculously so, but cyborg characters can't swim without floaters, and the military shells are heavy enough that they need reinforced furniture and special shocks for their cars. The cooler the shell, the more maintenance it needs, and all of them need some. Also, to have a cybershell, you need to have a cyberbrain, which opens a character up to being brain-hacked. It's not something most people outside of the GitS universe could do, but the risk is there.