Gas efficiency isn't that good, in my opinion. But... I do have an older FM KJW M9 myself.
I have no issues with the safety not working - however it doesn't work if the hammer is not cocked.
The feed lips are plastic, yes - but they are strong, in comparison to most GBB mags. Just note that KJ mags tend to develop leaks very quickly, and because of that, dropping them is probably the worst thing you can do for them; that should be your main concern, rather than plastic feed lips. Most GBB mags have plastic feed lips and not all of them have problems unless you severely abuse them.