If you play on a team, your MMR gets inflated.
Basically, your team's MMR's are averaged together, and then a flat value is added to it depending on how many people are on your team. Therefore, if in the unlikely situation that a five-man premade gets put up against five solo queuers, the five solo queuers are going to have a substantially higher average MMR than the team, and are likely to be more mechanically skilled than most or all of the people on the opposing team. Does this balance out? Is enhanced communication really better than being mechanically disadvantaged?
Well, duo queue was pretty balanced before. People were able to climb pretty well both doing solo queue and duo queue, and there were many proponents for each. Solo queuers got the benefit of having the average MMR of games, and duo queuers had the benefit of increased communication between eachother in exchange for one or both having lower MMR than the average of the game. Many people advised against duo queueing because they saw giving the enemy team a MMR advantage was too great a penalty just for having better communication. Now, imagine that MMR boost x4, give or take.
I think that solo queuers will be relatively balanced against multi-queuers because the MMR difference will be enough to counteract the advantage gained by better communication. Riot was able to balance solo queue versus duo queue well before, and I can trust them to balance solo, duo, trio, quadra, and penta-queueing as well.