Well, GTA Underground was in development since 2015, Underground Multiplayer started only a year ago based on SAMP 0.3.7 thanks to Kalcor and went into full development, it was previously based on 0.2x which wasn't good at all.
Let's see this from a realistic perspective, Every SAMP/MTA player wanted to roleplay in LC or VC at some point, but couldn't or didn't, due to the limitations in VCMP or Liberty Unleashed, basically the game engine not allowing a smooth and improved experience as seen in GTA SA's Multiplayer, and with GTA SA's engine, we have MTA and 0.3.7-DL, which allows you to actually have LC or VC in your server, but since it won't be running them natively, it causes stability issues.
Now UGMP only shares cores with SAMP and that's about it, I guess you could call UGMP "an improved SAMP". Beside letting you load all assets from GTA Underground, UGMP got a better syncing, more development features and natives and of course, custom animations.
You don't need to sit there for two years for a UGMP update to come out like SAMP, it is constantly in development and it's improved every day.
And it's still not too late, UIF gets 650+, LSRP gets 300+, RCRP gets 100+, but the way that UGRP is going to renovate roleplay as you know it will definitely be worth peoples attention, considering about 100 new players have joined the Discord Server within less than a week.