Been playing this again recently after maybe a year or so and holy shit is the matchmaker still awful.
Yeah, I quit playing 2 years ago because of this very reason. Recently returned and it doesn't seem to have improved much. I think it's less the matchmaker though and more just how the game "works". The biggest issue is that because HotS is such a team-focused game, just having 1 shit-tier player or troll essentially puts your team 20% behind.
Hots sure feels more casual on the surface but you're in for slap in the face if you're thinking of just waltzing to grandmaster. The fact that 1 player on your team can indeed fuck the biggest lead you have makes it way more brutal than it looks.