Ok, I'll try to answer it all.
First : to my knowledge there is no WE URX because G&P has the Knight license and they went with WA system. Also you need a special tool to install URX RAS (I have the RS one for gunsmithing).
Break ? Unreliable ? I don't know who says that but I saw a lot of GBBR both WE and WA, they are not less reliable than any other GBBR.
It is true however the NPAS of G&P is a little tricky, you need to put some break on the threading, easy to do.
But I had a LOT (big understatement) more issues with WE than WA, the G&P WOC-X series on ehobby is the best price/quality ratio I ever found. For suppressors it's a bit of a pain because they use 14 CW instead of 14 CCW.
On GBBR for race rifle all you need is trigger control (I can put 6 to 7 rounds on target in 2 secs with normal bolt carrier). It's not easy for a average player used to GBBRs, impossible to a rookie, have to train (I use RS to so I'm used to it).
The G&P tube is not milspec, it's still a special airsoft buffer tube. That's why you can mount RS PRS or UBR stocks on it without mods due to the dimensions of the buffer tube threading.
However for Magpul stuff it's using AR15 "commercial" dimensions so you may use a RS CTR or MOE or ACS or whatever. Just be sure it is made for commercial tubes.
Tous les jours nous nous endormons plus instruits mais pas forcément moins bêtes...
I can do a CTRL+ATL+DEL on your gun !
Gun doc in Montreal on AEG, GBB, GBBR and Bolt rifle (gaz and spring)
Heavy custom builder (including painting)