The UBR always kind of held a special place in the stock lineup. It was designed to be a brutal beast. The smaller PDW style stocks like you showed are just the newest thing on the market. In another 5 years, it will be something else.
I find that the new UBR 2 does not have a lock on the stock, so the bottom release tab catches on EVERYTHING and causes the stock to slide into its most open position at the worst times. I can just relax and let go of the rifle and the tab often grabs onto a mag and slide open, doing mag changes in unorthodox shooting positions on the push out the tab grabs the side of a mag and will release to full extension. Besides that I LOVE the stock, but I wouldn’t have bought it with what I know now. Won’t get another one until the fix protect the release tab or put a lock on the the thing. Really annoying and disappointing. Love Magpul, but I think the UBR2 is not good to go. YMMV. My .02