Given that I've been brought up as an example like 5 times due to having a good mono record, I feel sort of obligated to respond. I've played quite a few mono tours over the past few years and while I've found it enjoyable, I definitely don't think it belongs in SCL. The issue of matchup has been discussed at length, but the counter-arguments have been rather myopic. Monotype being matchup heavy doesn't just mean that you automatically lose if you bring Psychic vs Dark or Flying vs Ice. It's irrefutable that when you have six pokemon of the same fucking type, your weaknesses overlap. The majority of Kev's post is spent listing examples where type matchup itself didn't decide the game. What that fails to capture is that the specific sets chosen often (far too often) do. Just to illustrate this with a few examples from his post:
To summarize/reiterate my point, monotype being matchup-based doesn't simply mean the type chart. Monotype inherently has a very shallow pool of mons to use and because of that, has limited ways to deal with particular threats. That obviously leads to encountering unwinnable matchups far more often than you would in any other tier, as evidenced by the two most recent team tour finals.
King Choco vs Sabella [Psychic vs Dark]
Shiba vs Maki [Fairy vs Dark]
(Bo3 series):
Game 1 [Flying vs Dragon]
Game 2 [Ice vs Flying]
Literally every single game here was completely decided on preview. The post (fairly) describes how this was due to good prep, but ignores the actual issue entirely. Good prep in SS OU doesn't result in all 4 finals games being completely unplayable for one side or another. The issue with monotype isn't just the type matchup, but the very limited pool of what's available to each type and what they can use. That's precisely what makes it so easy to win a game on preview with good prep. Just because the type that won wasn't what the type chart predicted, doesn't make the game any less matchup-based.
Shiba vs Fírnen [Electric vs Dragon]
TJ vs Conflux [Dark vs Dragon]
Maki vs Padox [Electric vs Dark]
In this finals we've at least improved to 2/3 (the second and third) games being decided on matchup instead of all of them. Once again the unplayable games are explained away by ill-preparedness and good prep. While that certainly might have been the case, you cannot find 6/7 finals games across 2 major team tours being decided on matchup for literally any other tier that's ever been in an official team tour. Yes, people often find an advantage as a result of preparing well in other tiers, but the innate nature of 6 shared types and limited diversity exacerbates the issue.
tl;dr here is once again, neither of the games are the product of random pokemon/matchup losses. The team got complacent and reused teams and didn't take the effort to properly prepare for their games.
This tl;dr is what annoyed me the most. Arguing that monotype isn't random is missing the point entirely. It's not random and it certainly rewards good prep. The issue is that winning in the builder happens far too often either due to type advantage or sets that win by themselves as seen in 6/7 finals games.
I definitely don't want to get into the details of this section and keep repeating myself but yeah, type matchup is only the tip of the iceberg as far as monotype being matchup heavy.https://replay.pokemonshowdown.com/gen8monotype-1257241091 (Grass vs Flying)
https://replay.pokemonshowdown.com/smogtours-gen8monotype-533996 (Dragon vs Fairy)
https://replay.pokemonshowdown.com/gen8monotype-1268287697-jolr64xwzcx62rgx9826ev2r6iplxmbpw (Dark vs Fairy)
https://replay.pokemonshowdown.com/gen8monotype-1149818797 (Electric vs Water with no immunity)
https://replay.pokemonshowdown.com/smogtours-gen8monotype-511516 (Fire vs Bug)
https://replay.pokemonshowdown.com/gen8monotype-1265342938 (Ice vs Water, good for Ice in theory because of Freeze Dry being common)
https://replay.pokemonshowdown.com/gen8monotype-1262436038 (Flying vs Ice)
https://replay.pokemonshowdown.com/gen8monotype-1300720266 (Flying vs Ice)
https://replay.pokemonshowdown.com/gen8monotype-1254926638 (Fire vs Ice)
https://replay.pokemonshowdown.com/gen8monotype-1255106695 (Ice vs Grass)
https://replay.pokemonshowdown.com/gen8monotype-1145053197 (Fire vs Water)
https://replay.pokemonshowdown.com/gen8monotype-1255250960 (Ground vs Steel)
https://replay.pokemonshowdown.com/gen8monotype-1275552925 (Grass vs Water)
To summarize/reiterate my point, monotype being matchup-based doesn't simply mean the type chart. Monotype inherently has a very shallow pool of mons to use and because of that, has limited ways to deal with particular threats. That obviously leads to encountering unwinnable matchups far more often than you would in any other tier, as evidenced by the two most recent team tour finals.