My daily ride is GX, but my only experience with NX has been rentals, demos, buddy bikes, etc. One thing I noticed was NX didn’t handle loaded downshifts as well. But coming from riding XT for years, I may be pushing it a little for both NX and GX. Although, my GX does OK trying to downshift while on a steep technical standing climb. Other than that, I don’t have a lot to go on for comparing GX/NX. Having said that, I didn’t notice any real issue with NX in my limited experience with it… works as advertised. I don’t measure grams, so have no idea about weight differences.
Maybe someone else can expand on their NX experience, but my thoughts on GX are (coming from XT): I’ve got close to 10 months on GX now. 2018 model. The usual abuse, including at least two (that I remember) rock slams causing pretty decent gouges into the derailleur’s aluminum body. Stuff gets an occasional cleaning and oiling once in a blue moon. Loaded downshifts work pretty good considering the angles needed to reach the span of a 12 speed cassette. Upshifts are quick and solid. I’m not crazy about the shifter. Thumb press downshift, index finger pull upshift (only). It works fine, but I prefer the two way shifting using either finger like XT does. Even with slamming the rear derailleur into rocks, I haven’t had the need to make any adjustments. It’s held true, including throughout break-in. The only issue I had was one of the idler gears nearly seizing. Had to take it off, clean it out. All in all, no real complaints. And to be honest, works as well as XT. Just different ergonomics. As for wear… I don’t see any unusual wear patterns that wouldn’t be expected.
Hope this helps.