The closed queue of 6s that occurred - at least in EU - was mainly good, since pugs were high quality and everyone tried to win because of the points. Everyone would join voice and try to win, it was locked to anyone who was mid+ so the games were very high quality with most games being filled with prem/d1/d2 players.
But when the system got pushed into open queue, it was a trainwreck for lots of players. The system got really bad for mainly those mid and below, though some higher level players got affected too. This was because for the mid level faceit ranks (mostly <7 and below, sometimes higher ranks) got matched up with pubbers who ran pyro/heavy/engie to mid and didn't join voice even when players tried to send discord links in the faceit browser chat.
There was some bandage fixes of giving players elo adjustements in the discord and matchmaking changes, but the damage was already done. There was mass negative criticism due to the awful pugs and after the testing ladders finished + some surveys, the whole of faceit 6s matchmaking was dropped into limbo.
Hopefully, Sigafoo and the team at faceit are able to use the feedback provided from these testing periods to create a fun 6s matchmaking platform. But the future doesn't look too bright as of now.
TL:DR faceit has been avoiding 6s and focussing on prolander because of the massive negativity that occurred during the 6s open queue test.