Flex queue has the worst matchmaking hands down. One reason for this is that to match a 3 man premade, you need a duo or 2 solos. Most duos go to solo queue which means its really hard to find a reasonable match for 3 man premades.
I'd say "duo should be removed from solo Q" but iirc the game either 1) matches you against another duo or 2) the team without the duo gets better MMR players
Flex is for playing 5 man ranked, I play it instead of ranked teams, but somehow still feels like the same when it matches you with dia+ players in gold.
As for solo/duo queue it was always solo/duo and to be honest it is better to duo there than in flex, simple because flex is dynamic queue just being renamed and people remember the terror of playing it with a duo partner.
