The point of the RXM is making customizations easier by making the FCU and not the frame the "firearm", like the P320. Seems like a decent option for somebody who wants a cheaper Glock with more frame options. (Okay, there's like 4 colors and 2 sizes so far, only Magpul and maybe one other company is making frames) But for 3D2A peeps, if you want a different G19 frame, just print one. As burble said, the advantages of an FCU don't really apply to the 3D2A space, since we are already fabricating the "firearm". There are a few RXM designs on the index but it doesn't seem to have taken off. FWIW it does seem to be taking off in traditional gun owner circles though, and appears to be a decent gun in its own right.
Can't speak on Urutau builds, but the Ballistic Advantage barrels are often recommended for blowback AR9 builds, and the one I have in my AR9 with a Hoffman lower works great.
I've only used Aves rails so I'm not sure how others compare. But the front and rear rails, and the locking block, from Aves have all worked fine for me. Another option could be bedding the front rail with epoxy or something.
I've had similar issues with front rails being loose. It seems like most designs just have a bit too much clearance. Side to side play can be fixed by shimming with pieces of metal from soda cans. One layer shoved between the rail and frame on either side of the rail. For the rocking I've reduced that by melting small areas of the frame plastic with a hot nail to fill in the gap between the front and/or back of the rail and the frame. Like staking an AR-15 castle nut or bolt carrier if you're familiar.
As far as the pins walking, I've ended up using a size smaller drill bit (whatever imperial size is just under 3mm) for cleaning up frame holes to keep them from getting too big. Then dry fitting the pins and drilling the holes a bit more until they fit snugly. You could also stake the front rail pin in using a hot nail like the rail. Or a dab of super glue.
Very cool! A simplified version for stickers might be cool too