Splatoon

No one but John Nintendo actually knows for sure how matchmaking works, since that's all server-side. We have some idea of parameters it looks at - rank, Glicko-2 rating, weapon range, location/ping, how long you've been waiting in queue. But there's no way of knowing for sure what the server does with that information, how these parameters are weighted and prioritized against each other.

Based on what I've observed though, I think it is likely that only Series cares about letter rank (some people even think Series doesn't either, but based on how Rank-Up Battle works I don't think that's correct), Open doesn't appear to pay much attention to it. Letters are fake anyway, there's a second internal rating being tracked: Glicko-2, which is a much better algorithm.

When Open was reworked a little while ago as a replacement for League, Glicko-2 is now visible when playing as a group. Press L during matchmaking and you'll see "Calculating: X/5" for the first five games you play in a rotation, then after 5 games it'll show your rating. Note that you only get to see it when playing with a team, and if the team changes it'll take another 5 games to become visible again, but we know it's always being tracked even when it isn't shown. It's a pretty safe assumption that this is what Open cares about, moreso than letters.

One important detail to note here is that it is calculating a rating for your team as a whole, which is why it goes back to "Calculating: 0/5" if the team changes. Even if there is a skill gap between teammates, that can be balanced by tracking how you perform together.