I know it's become a meme to talk about how unbalanced UU is right now but like... are people actually playing the same tier that I am? Since the last round of bans I've found this to be one of the most fun metas since SM's release. A pretty sizable percentage of the complaints about the tier's balance that I actually see tend to shake down to "I don't like playing against VoltTurn" or "I don't like having to prep for top threats." The last tier shift definitely made things fairly rocky, but things have settled a lot following Jirachi's ban, and the Weav/Garde bans have served to really balance things a lot as well. I'd certainly place the current meta well above the Jirachi meta, the AloTales/Lucha/Xurk meta and the Conk meta. People have taken to waxing rhapsodically about the Clef meta lately but when it was actually legal we got a ton of complaints about how bad Clefable was for the tier.
Since the Gardevoir/Weavile ban, balance and stall have both become significantly more viable without diminishing offense as a playstyle, and despite the complaints about VoltTurn (which banning MLati will do nothing to alleviate and will only make worse) I actually see a ton of variety both in tours and on the ladder. This is probably the most balanced meta I've seen since around UUPL, and while I know fun is subjective, I definitely have been having fun playing it.
Also re: the speed of suspects, the tier shift was the beginning of September. It’s not yet November and we’ve already had six bans. I understand that it’s frustrating to go through a suspect process for something “obviously” broken but I don’t know of any other official tier with as many bans in as short a period of time. Personally as TL I don’t want to fall into the trap of banning things just to preserve the status quo, which is why I prefer incremental suspects to mass council bans.
As for Latias-Mega itself, I'd agree with Hikari that it's probably the best 'mon in the tier right now. It has incredible defensive utility while also consistently threatening a late-game sweep, akin to Suicune at its best last gen. That doesn't necessarily mean it's broken, though. It's certainly something you have to prep for, and you do typically have to prep for multiple sets—but the same is true for Scizor and regular Latias, and the same was true for Clefable when it was legal.
Right now I think that MLati's usage largely breaks down into 3 main roles: defensive utility, lategame sweeper and hybrid. To break it down further...
Defensive utility is where I'd place Defog versions, Thunder Wave versions, etc. It's also (ironically enough) where offensive variants like 3 attacks + Roost tend to fall. These are sets where Latias relies on its excellent bulk and coverage to switch into a large number of threats and force them out, get rid of hazards, spread para, whatever. These are really consistently good at what they do, and probably the sets I use the most frequently.
Lategame sweepers sacrifice utility and offensive presence for the ability to sweep through teams lategame. These usually either run Stored Power to break through bulkier teams and special walls, or Reflect Type/Refresh to become less vulnerable to status and Dark types. They're definitely deadly, but they lose out on immediate firepower and become far easier to pressure offensively, both because it's much easier to switch directly into these sets, and because their lack of immediate firepower means that they often don't threaten out whatever they've come in on. For example, while I often use MLati as a solid Cobalion check, I managed to beat a CM Stored Power MLati with my own Cobalion last night because it had no way to 2HKO me in return, giving me the opportunity to Swords Dance and kill with Corkscrew Crash. These sets are incredibly deadly and difficult to stop once they get rolling, but they also require a significant amount of team support and lose out on a lot of the utility that Latias otherwise provides.
Hybrid sets try to do a bit of both, relying on two decently strong attacks with good coverage such as BoltBeam or Dragon Pulse + HP Fire, while still sporting Calm Mind to help it better check opposing special attackers and potentially sweep. These are a nice compromise because they retain enough utility to be able to fit on most teams without a ton of support, but they're also generally the most vulnerable to status, and they often struggle to break through dedicated special walls like Blissey.
Most of the arguments I've seen for banning MLati fall under the second category, which admittedly is scary; there are only a handful of things in the entire tier that can actually outspeed and kill a fully healthy Latias-Mega, so if it racks up a couple of boosts it can become very difficult to deal with. What these arguments seem to ignore is the team support, luck and prediction required to get these sets to work. For every game where I see Latias-Mega do something ridiculous like set up on a Hydreigon, I see another two where it gets trapped in a cycle of endlessly roosting to remain healthy enough to set up until it eventually gets forced out or crit. For every game where I've seen a surprise Reflect Type beat an otherwise solid counter like Alola-Muk or Toxic Empoleon, I've seen even more where being forced to a single attacking move meant that Latias wasn't threatening enough to sweep even after several boosts. It's definitely good—I think there are very few threats in the tier scarier than a fully healthy Latias-Mega with a Calm Mind under its belt—but it's just not so good that I would call it broken.
If there's any argument at all for Latias-Mega being broken, I think it probably falls under how difficult it is to prep for all the different varieties it presents. I don't think I agree with this argument, and if we voted today I'd be voting no ban, but of all the arguments I've seen about Latias-Mega's negative effect on the tier it's the only one that really has merit for me. It's what I plan to focus on for the next week and a half to see if it sways my opinion.