SBMM is and always has been a joke. That's what ranked play is for. Pubs should just be a free-for-all of randomly filled lobbies.
This season is particularly terrible for SBMM. Typically I get to Diamond every season then stop ranked play, but have grinded to Master in a previous season (not fun to do). This season, 9/10 of my pub matches are against 3 stack pred/master teams. May as well be playing ranked, why even have the distinction between the two? My win rate, which has averaged 20% since the game launched, is down to 12% this season.
Between that and dying behind closed doors or around wall cover, this season is the most frustrating for pub play.