As much as I'd love regional matchmaking - and have been saying so since the release of the game - it is a concept that is better in theory than in practice. If there were enough players of every rank in every region, and everyone had great internet connections, it would be fantastic. But that just isn't what we have. Experiences during Splatfests probably aren't good examples, considering that the servers always seem to have more issues than usual at those times(which shouldn't be acceptable, but might explain the problems at least), but we have all experienced players from your own region that have been very laggy, even outside of Splatfests.
So the first con of regional matchmaking is that it wouldn't solve as much as we wish it would, as regions is only one link in the chain of issues that lead to laggy games.
The second con is that it would likely affect the pool of players a lot, which could be a huge problem for European and Oceanic players in particular.
We all feel like 9/10 players are Japanese, and from what I've heard, around 8/10 of the remaining players are American. I don't know how accurate that second number is, but as a European myself, it certainly doesn't seem unrealistic, if judged based on how much fewer European and Oceanic players are seen on these forums. And just how much truth there was to this, was first experienced two Splatfests ago, when Nintendo had their second worst idea in Splatoon's history: scheduling the Japanese and American Splatfests simultaneously. (The worst idea being to NOT change that concept for the next Splatfest.)
EU playerbase at least has enough to support it. Since the splafest dates got changed, EU has had 2 days without either Japanese or NA players and it's been fine. There hasn't been excessive waiting, sometimes a little longer, but nothing excessive. That's S/S+ solo, can't speak for squad ranked.
My friend also had a quick S/S+ solo queue, but guess what? Before he queued for ranked, I queued for Turf War. He had time to queue for ranked, find a game, play the game, quit, idle for a minute and then join my Turf War game before we had found eight players. And that's not all, we also tried to play squads. Twin, tri and quad, for between four and five hours. In that time, we got to play less than ten games. We had long periods of having nothing but queue timeouts.
Our region is fine during our own Splatfest because everyone is forced into the same game mode, and a lot of people want to play the event and get rewards. But otherwise, we're not fine ... Now imagine if our numbers were spread
even thinner because we also got to choose betwen regional and worldwide matchmaking. My guess is that most people - in order to get faster queues - would opt for worldwide matchmaking, which would mean that regional queues would be veeeeery slow(compared to what we're used to)due to the low numbers. In EU/OC. In Japan, it would of course be a different story. They certainly have enough players to maintained regional matchmaking for themselves, so why should they queue worldwide? I'm sure "a lot" still would do it, but obviously the numbers would be lower than they are now, which means that EU/OC would have an undercrowded regional matchmaking and a worldwide matchmaking with a much smaller player base than we have now. That's not a situation I would like to see happen.
So as much as I would love the option of regional matchmaking, if it worked, I don't think it's a good idea right now. The game needs more players first. Making queues longer could potentially deter people from playing the game, and fewer players is the opposite of what we need to make regional matchmaking a good idea!