Funnily enough we were doing our placements last night and noticed something interesting (anecdotal of course)...
The first 5 games we played there were 4 of us on; 2 low gold players and 2 high plat, so quite a spread. Those games were pretty bad - all losses, and in most if not all of them we noticed the familiar sight of one or two players on the other team either being obvious smurfs (e.g. low level accounts but with insane seeming skills) or just higher rank players queuing with lower ranked friends (which to be fair was like us)...
After those games one of our gold players left, and we won the next game - it felt too easy and we rolled them. Then that player left leaving just 2 of us - both plat, very similar SR actually. The 4 games that proceeded were all wins, but they were very close games - no complete stomping on either side, just very hard fought games that felt balanced...
This makes me wonder if perhaps the matchmaker does take the spread of any groups into account - if you've got large disparities in SR between players in your group, you should expect to be matched against other groups that have a similar makeup - and in a lot of cases this could put you in unlucky territory (especially if your higher ranked players aren't all damage heroes that can carry and the other team's are)
From my experience this is definitely the case. Average SR and make up of both teams will be similar, and I'm often in games with 5 gold and one plat on each side, or 4 and 2... You really want your plat player to be DPS! That said, I've seen @imginy carry with Brigitte, and I've seen a couple of Ana carries, but generally the team with the plat DPS wins.