There are a multitude of reasons, so I'll just state my own personal one for disliking it.
I feel ranked should be an even playing field for all players. Every player should start the game with no inherent advantage or disadvantage compared to the other 9 players. Premades clearly have an advantage in communication. Even when both teams have them (which riot claims is almost always the case when premades appear at all), that means that there are still one or two solo players on each team who are at a disadvantage compared to their enemies and their own teammates. My main issue is that playing WITH a premade often puts you in the backseat while the premade decides the game, due to how strong the edge in coordination is. For a game that is supposed to be about competitive skill, giving any of the players an advantage over the others is unacceptable to me.
I would like to see pure solo queue, where not even duos are allowed... and five man ranked teams. And that is it! These are the only two modes where I feel all the players on each side start the match on even footing, with no one having or lacking something that other players in those games have. My issue with Dynamic Queue is that it isn't fair... and I don't feel it CAN be fair. I think Riot is just going to assign some MMR bonus to premades or penalty to solos and call it a day if the numbers seem to be close to "each group wins 50% of the time". But those statistics are misleading, because those games aren't really even, they're just skewed to look even.
I also just personally don't find it fun to take a backseat to four-man premades whenever they show up, but that's a personal issue. The mode just feels unfair to me, because it provides an in-game advantage to some players while letting others suffer under the typical solo queue problems of poor coordination and team communication. There is NO advantage to it for solo players... and it feels to me like Riot is playing favorites with which players they care about.
