oh hey I meant to respond to this too now that I can do so properly
I agree with earlier posts that the gen 3 frontier is the hardest from an in-game achievement / Trainer Card level lens, but that's also speaking almost entirely off theory and the common sense of seven gold symbols being a steep requirement, I don't actually have experience with those facilities other than grinding some quick BP for a couple tutor moves with in-game mons. In generation 4 I ended up getting the 100 wins for the Trainer star in Platinum plus all the non-Factory gold symbols; I got my 100 streak on my second attempt but did find the roster pretty difficult to handle compared to more recent facilities since a ton of enemies run the entire rainbow of elemental coverage plus hax-focused sets are more common than later, not to mention that of course this is the longest streak requirement any game puts out for a Trainer Card upgrade which inherently adds to the difficulty level compared to elsewhere. The Factory is inarguably the hardest facility in the franchise, but the other Frontier facilities are not really relevant; gold print Hall is pretty free if you use Garchomp like "everyone" does, and (unless you're running a Trick team I guess due to those not functioning outright without their items) Castle and Arcade are close enough to "Tower with an asterisk" at least for a simple gold print run that you'll be just fine there with the same team (I think; at least I was).
For longer streaks, I can comment on gens 5-7 and BDSP. Subway/Maison/Tree go together in the sense that the rosters for the more recent ones are edited/updated versions of the Subway roster rather than redone from scratch. To get the Maison out of the way first, there's two big dynamics here affecting difficulty level, namely players having access to Megas while the AI doesn't, and also predictable enemy rosters, in the sense that other than legendary Trainers and a few specific classes the vast majority of the opposition will use only one set, which after battle 40 will also be only the fourth set out of the roster for every species. The Megas thing is a big reason why people say the Maison is the easiest facility of these three, which obv nothing to argue there; the same goes for the lack of set ambiguity, but I should add that this is more of a thing that makes longer streaks easier when it reduces the pool of Pokemon to prep for in the long run when the other sets stay contained to battles 1-40, so for a simple trophy run it's more of a wash.
The Tree is really weird to assess here. There are a handful of factors that make it harder than the Maison, namely the presence of Megas and Z-Moves, and I'm also working on a post elsewhere rn where I'll be mentioning that "every Tree Trainer is a speciality Trainer"; one thing here is that the very vast majority of them will be running sets 3 and 4 for every species by default (though iirc that's less of a thing in the earlier rounds so once again more relevant to longer streaks than to the in-game achievements), and there's also a heavy increase in e.g. Trick Room specialists as well as new archetypes such as Trainers that only run Pokemon with (very) high Speed stats. This makes it much easier for the AI to roll multiple threats especially to shakier teams at the same time where in Maison (and Subway but I'll get to that) such clusters were less likely statistically, making it a more punishing experience overall. From a Doubles lens specifically, the AI was also changed to make its moves harder to predict, which comes with a few issues as well. The Megas / Z-Moves point does warrant a few more words, since while people commonly say they level the playing field, that's of course true to an extent but at the same time we still have access to the Tapu / Ultra Beasts / USM-only Z-Moves, which the AI does not, and a look at the Doubles leaderboard (read: 95% of 200+ teams use at least one of these) should make it clear that that's actually a really significant leg-up for us. A better way of putting it would be that it makes for a massive powercreep that results in the player-exclusive elements being all but mandatory in Doubles, which is exacerbated by the occasionally drunk AI forcing you even more into using the strongest elements possible to keep a leg up momentum wise, and Singles being extremely hard to build a half consistent team in, because thanks to Tapu and friends being much more middling here we no longer have the same advantage in tools over the AI. The Tree is certainly difficult, but it's much more in the builder than elsewhere; actually on the field the singles AI in particular is benign enough to read and play around, hax sets are probably less prevalent than ever and crits having been nerfed twice is something that can't be understated, and even with the Maison Doubles board being rather obviously undercooked I'm quite convinced at this point that even with the extra advantages we have there a "good" Tree Doubles team should still put up more or less equivalent scores as a "good" Maison team, of course in no small part due to how good Tapu Koko and Z-Kommo-o et al really are. Just, "good" teams over here are harder to put together in the first place.
The Subway is the one facility out of these three that gets closest to putting the AI and us on a truly level playing field, with HAs being the only real advantage we have over them. Roster-wise, it strikes an (imo) great middle ground between Maison and Tree, where Trainers with set ambiguity still do exist and also do run all four sets of their species, but with the majority of them we do know exactly what we're facing, except there's also Trainers running set 1 or set 2 or set 3 exclusively, meaning the old sets are never taken out of the rotation. There are a few specialty Trainers with very threatening rosters (Bikers, Sand Workers, Ice Workers) but the fact that A Lot of the time we do know exactly what we're facing and are not forced into midground plays helps a lot with pacing and makes it really pleasant to play. The main thing worth mentioning about the Subway is the "illusion of fairness"; optically it's a genuine good attempt at putting up an actually engaging challenge between us and the AI (rather than being focus on haxing us into the ground) where we also have a good range of options we can choose to run ourselves, but certain old mechanics still have a habit of ending runs earlier than they should and making us lose battles we have no business to, think double Protects having a 50% success rate and crits still being 2x damage. It's a reason why there tends to be major variance between scores and why it's really not out of the ordinary for a really good team to fail to break triple digits on back-to-back runs. From an in-game achievement pov specifically, it's also worth mentioning that the "real" Subway starts pretty early; while Maison and Tree wait until battle 40 to roll up with the rosters we'll be facing into perpetuity and are running 31 IVs across the board, for the Subway this too happens after 4 "sets", except this time that's as early as battle 28.
The BDSP Tower is entirely different in the sense that the teams we're facing there are fully preset, which makes for a weird dynamic where you're actually building to counterteam individual Trainers (especially Palmer, who appears every seven battles on long streaks). This has its ups and downs, since of course it gives us the obvious leg-up that we can plan around their backups as well and don't actually have to preserve our Pokemon once they're no longer useful in a matchup; on the other hand, while attempts to make enemy lineups synergistic are hit or miss, this does mean that you really do have to beat everything no questions asked or you're just playing catch-up with the lineups you do lose to. The last point is what it almost always boils back down to, since in practice this ends up being a more extreme version of the narrow roster threat stacking I mentioned with the Tree; while in conventional facilities you do get a long way by just hoping x scary threat won't be paired with another threatening enemy, over here, well, either it is or it isn't no two ways about it and you can bet the creators have at least tried to make it so, and if you're not properly equipped to deal with certain teams where they do get stacked then it's a matter of time until you lose. In Doubles, a few teams have actually gotten close to solving every matchup like this, but in Singles, where you have fewer tools and the creators probably did the best job creating demon teams anyways, there's extremely few people that can even begin to make that claim, to the point that I'm quite comfy calling BDSP Singles the hardest mode of these facilities outright. The good news is that for the Trainer Card level you can also play in the regular mode that is mostly just used to unlock Master Class and is all but unloseable with a good team, but yea for ribbons and for grinding BP Master Class is probably where you should be.
thanks for bringing this one up, I have for the most part not directly answered it oops since I think a lot of the time it's also a matter of perspective but thinking through all the dynamics in play everywhere is good!
