Control SBMM currently averages out the skill of your teammates.

Some people are actually using this to manipulate the matchmaking to get easy games, unfortunately.

Like look at this player:

https://destinytracker.com/destiny-2/profile/bungie/4611686018445196435/overview

He had a 10 KD in Control for this season. He's cracked! Right?

Well, no, he's using three different stat-tanked smurf accounts to force himself into games against much lower skilled players, here are the links to the smurf accounts he uses:

https://destinytracker.com/destiny-2/profile/bungie/4611686018525245459/overview

https://destinytracker.com/destiny-2/profile/bungie/4611686018525225984/overview

https://destinytracker.com/destiny-2/profile/bungie/4611686018525592275/overview

If you look at the main player's history you see all the Control games he plays against players who are much, much lower skill because of the SBMM manipulation. So this kind of exploiting sucks, but at least it proves that the fireteam skill is actually being averaged out with SBMM, it isn't tying it to the highest skilled player as some are incorrectly stating.