Theres a couple factors why this may feel like the case.
First, if you and your duo partner aren't extremely close in mmr, it's going result in being matched with/against people around the average of your mmrs, so players are going to be worse than the better of you in theory.
Additionally, it'll try to match you against a duo as well, who again might have their own mmr mismatch. While both sides might have e.g. a gold 1 player, a gold 5 player, and 3 gold 3 players, the lane match up could very well end up with the gold 1 stomping on the opposing gold 5 and getting quite fed off it, while the other gold 1 is facing a gold 3 and not doing quite as well. The particular roles can also make a difference too, as some roles are able to have more early impact than others.
Also, theres lots of kinds of duos. Some people are just two casual friends who like playing with one another, others are two more serious friends on voice comms and more actively calling plays, and of course there are duos where one person is a smurf with skills well beyond their rank, and depending on which category you fall into, you'll need to be wary of people from later groups occasionally popping up.
If matchmaking doesn't find a duo to match you against, it does give your teams mmr a small bump to compensate for the extra communication advantage you have, so the enemy team will be slightly stronger in individual skill levels on average, but since they don't have the extra communication it should be 'fair'. The solo players might find themselves slightly outmatched though, and if you're not actually leveraging your duo comms, you may find yourself individually outmatched too.
How they 'should' matchmake premades is debatable (some feel they should be matchmade as normal since they're just playing with friends and not being super serious, others feel there should be artificial mmr adjustments to account for comms so it's not just 'which team has the better premade'), but they've tweaked it a lot over the years and feel like they've got things adjusted so it's roughly 50% winrate instead of premades having a clear advantage or disadvantage compared to solos.