I have done work on 3 metal bodied jg mp5s. Mine was luckily very good. No scratches or tight spots. The other 2 weren't quite as good. They were scratched where the upper and lower receivers overlap and one had loose body screws.
Nothing that would keep it from working fine. Just cosmetic.
This was over a year ago and the recent jgs I've seen have been even better.
I'm using stock mechshell, spring and gears. Most other internals beefed up.
Even with mini battery and 400fps +, this thing runs smooth all day without recharge.
Not a high rof but I have no complaints. I sold my marui and now use jg and echo1.
The paint on the metal body will start to wear thin around the grip and any other high-rub areas. Just looks field-mean.
Mp5s are hard to disassemble? I'll take it over any other aeg. I like the split-body.
Grand Poobah of T.W.A.T.