Yeah, team based matchmaking is such a god damn nightmare the more you start thinking about the problem. Especially in a game like Overwatch, where a team needs people slotted into different roles.

You have how many people looking for a game? In groups of varying sizes, with varying ranges of min/max SR in the group. So you try to find approximately similarly-sized/ranked groups, filling the gaps with comparatively ranked solo queuers. Not too bad.

Now. Do you start looking at characters played? It'd be a total matchmaking failure for all involved to group six people who only play Mercy against a team of people with either more flexible history or rigid but diverse history.

So you're mixing and matching, hoping to find a good fit of players with relatively compatible skill and play experience, across a huge pool of players, and it has to be fast.

At one point well after release, Jeff said they had people dedicated solely to the matchmaking, and I believe it. It's not an easy problem.