It's rude of me to ask, given that unbans have literally just happened, but could breloom be unbanned and retested in the near future? It's one of the only BL mons I can't see as being broken because honestly if people dont see serperior as broken, neither is this thing. Just something to think about once we've deemed the tier as balanced and stable.
Serperior was banned in UU Beta on the basis of its prowess as a lethal SubSeed user and sweeper. It had limited counterplay due to its ability to easily garner a +2 Special Attack boost through merely clicking on a 130-base power STAB attack. Its coverage options allowed it to adequately take on the vast majority of the metagame on its own while still making room for tools that could be used to cripple what few Pokemon could beat it, like Glare and Taunt. The sheer success it had through its SubSeed and sweeping sets alone caused it to warp the metagame around it due to the lack of means of reliable offensive or defensive pressure. However, as the SM UU metagame moved forward, it brought trends that favored the possibility of Serperior being healthy. Amoonguss was quickly established as a premier defensive threat, capable of stomaching even a +2 Hidden Power Fire quite easily. Alolan Muk and Mega Aggron were also introduced to the metagame and are now established as reliable means of defensively handling Serperior. Pokemon faster than Serperior that weren't previously legal, like Mega Beedrill, Mega Sceptile, and Mega Manectric, were newly introduced and are now proven to be good revenge killers to it. Pokemon that previously suffered from too many metagame trends working against them to justify using on teams without significant opportunity cost, like Moltres, Choice Scarf Chandelure, and Crobat, began benefiting from changes occurring within the tier, meaning that they had more worth than as Pokemon that merely stopped an overcentralizing force. Serperior became easier to stop due to a gradual accumulation of things working against its dominance, those things including the establishment and introduction of reliable defensive Pokemon to hold their own against it, the release of fast and reliable revenge killers to the metagame, and the drastic metagame changes that allowed already existing stops to Serperior to become worth using for more than just functioning as a semi-sturdy Serperior check. Multiple team archetypes now have some ways of dealing with it (an example being Sap Sipper Azumarill seeing increased usage on stall), and that isn't something that could have been deemed true at the time of Serperior's ban. Serperior previously required minimal team support to do its job, more or less getting the job done on its own, and now it's considerably easier building in preparation for it and you have to make a more strenuous effort to support it.
Now let's move onto Breloom, and specifically why I think it should not be let back into the metagame, especially if it's wanted back on the basis of Serperior being allowed. Breloom's ban was on the basis of the fact that both offensive and defensive checks to it were limited. Teams that didn't have a Spore switch-in were automatically disadvantaged in that Breloom could easily cripple one of their Pokemon and limit switch-ins with its offensive presence from there on out. Its STAB combo and access to moves like Bullet Seed and Mach Punch in tandem with Technician allowed for massive damage to be done to a ton of Pokemon commonly seen on offensive builds. What's more is that the few Pokemon that resisted its stab combination, like Chandelure and Latias, couldn't come in for fear of getting nailed by Technician Rock Tomb, which assuming it hits, cleanly 2HKOes Latias on the switch given that the speed drops will then allow Breloom to outrun it. Another Breloom strategy that has been more explored following its ban that would've furthered a ban argument is that it can viably run Natural Gift with a Ganlon berry to break past any of its traditional checks. Natural Gift Ice would break past all of the Pokemon that you could reasonably argue kept it from sweeping entirely, those Pokemon being Amoonguss, Celebi, Latias, and Togekiss. This was overall enough to give it the ban hammer, since it was just too much for the metagame and only really required Pursuit support to function to its full potential, that kind of support being quite easy to come by. As the SM metagame has progressed, nothing has happened that would help Breloom's unban case unless it was counteracted. Buzzwole, a Pokemon quite capable of checking Breloom, was introduced only to have been banned a few weeks later. The new defensive Pokemon that came down, such as Mega Aggron and Mega Steelix, did nothing to strengthen defensive teams' matchup against Breloom, instead worsening it. Pursuit became an even more accessible tool with Alolan Muk's entry into the tier. Newly introduced Pokemon that could reasonably stop Breloom, such as Mega Altaria, would fall victim to either Rock Tomb or the Natural Gift Ice innovation. Breloom was banned on the basis that it lacked offensive and defensive counterplay sufficient enough to prevent it from substantially contributing to a team before dying, and that fact hasn't changed.
Basically what I'm trying to say is that despite their similarity in typing, Serperior and Breloom had completely different responses to the metagame trends that occurred throughout SM, with Serperior coming out as the healthier option to release to the metagame. None of the defensive Pokemon that were introduced to the tier since these Pokemons' bans benefit their team archetype's matchup against Breloom, while doing so with Serperior instead. Many of the risen Pokemon that can now deal with Serperior, like Crobat and Moltres, can be exploited by Breloom more easily in one way or another. Supporting Serperior became more strenuous of an effort, while supporting Breloom became almost easier. Breloom's presence in the tier would, in my eyes, be as unhealthy as when it was first banned. Breloom is a Pokemon that I've heard a lot of people saying they'd like to see given a shot down here, though I don't think that any metagame trend it exploited in beta has drastically changed enough for it to do less damage than it did before. Breloom's way around pretty much any means of checking it alongside the fact that it already exerts an extreme offensive presence would not bode well for UU if it was to see an unban. I think that to ask for its reintroduction on the basis of Serperior's reintroduction is quite frankly silly when in reality, they are drastically different Pokemon with different countermeasures and effects on the metagame.
I didn't mean for this post to act as a direct response to you Machiavelli, rather one that combats the common sentiment that I see that suggests Serperior's healthiness in the metagame only means Breloom could be manageable here as well. I hope this post didn't derail discussion, because I feel as though Breloom is enough of a hot topic for so many people that something like this could be worth discussing more thoroughly. Thank you to those that took time to read this.