Bear may be good at tearing through trees, but he requires remote storage, and is very difficult to use outside of autumn due to his massive yawn range. Merms let you have your trees right next to base and let you perform year-round harvests. The utility of bear uprooting stumps is nice, but you can easily dig up stumps while merms chop, and any extra time it takes is made up for by not needing to walk over to your dedicated bear storage spot. Forest walker is alright, but can only harvest at night and you can't dig up the stumps while leading him in an efficient manner, making a full harvest a little more irritating, to say the least.
Thermal stones require regular heating up, and don't even last a day without re-heating. Sunfish last for five days and can be swapped instantly. If you don't appreciate or want that utility, that's on you, but IMO that is a massive upside in terms of practicality, you can walk for 2.5 days uninterrupted with just a lantern without stopping, and have time to get back to base afterwards. You literally only need to swap three times (more like four realistically speaking ofc), during the entirety of winter. If you're trying to do some sort of super fast run of the game, then this won't be of much use, but for long term general play it's a really really useful ability to have.
In terms of combat - Maxwell has half of what wurt does without merm king, as does wes (who also does less damage). Being about equal to the joe schmoe of wilson isn't exactly anything above C tier, but I think having 250 hp does put her in B for durability alone. I'm aware other chars have minor combat perks, but for the most part those are very niche, with a lot of those being just static entities with an extremely high resource cost, not much to give an edge in notable fights for the most part. Catapults would probably be the best bet, but unfortunately for Winona, they can be operated without her, which really limits any sense of them making her any better at combat.
Merm guards are better spider farmers than bunnymen because they don't aggro you for picking up monster meat like bunnymen do. I never mentioned them being better for anything else, although in the case of bee queen they do have the benefit of a higher max dps due to their higher damage + kiting meaning they don't clog up the available space to hit her after attacking, plus they ignore the roar and don't run away when on low hp. Still, setting up enough merm guards to do that would only be worth it if you're full on megabasing and not just playing long term.
There is absolutely nothing tying you to the swamp either. Mermhouses only cost 3 reeds, meaning 30 reeds will get you enough for 10 mermhouses, and you really only need 5 to do all of your logging. And 30 is something you can easily collect within the first few days (assuming decent luck at finding the swamp). It's a nice arena for deerclops since it gives you a huge area with a road speed boost, but you can base anywhere you want by just digging up a couple of swamp tiles and building your mermhouses whereever. And by the time you're looking into amassing an army you won't need reeds at all, since loyal merm guards can be obtained from king trading anyway. The only real prerequisite is a pond, which is something you should be near anyway for the sake of a farm.
Also, having low max sanity is not an advantage at all. Losing sanity isn't even remotely difficult, and having a high max sanity means you stay insane longer, since your sanity threshold is a lot higher. If you absolutely must be insane you can just make a lazy deserter and drain your sanity in no time. Or stand in the dark for a bit. Or chug spoiled food. Or hold a dead fish in your inventory. I've never found myself short on nightmare fuel.
Leaky shack is mediocre for her early game, because while it gives you plenty of merms - the swamp is often well away from where you'd actually want to base. They're also not as consistent with leaving their home, and are often far away from any good pine area (although this can vary wildly). They're nice for getting up your initial log supply, but nothing essential IMO. Of course this one is heavily affected by worldgen, so your mileage may vary.
You don't need to be a megabaser to want a lot of wood either. Just having roads from one place to another is a really nice thing for a long-term world where you want to beat a bunch of bosses and are not concerned with doing it as fast as possible, and cobblestones will burn through logs like crazy lol.
When I say S late game I don't mean S tier in combat, I mean S tier in utility. Being able to ignore temperature and rain almost entirely is a really really useful attribute to me that makes her easily one of the most efficient "do stuff" characters long term, since you can make much longer and more uninterrupted trips to different places in your world, and you can stick to using helmets all the time without having to worry about an eyebrella for heat or rain. That's why I mentioned the max hunger - it's not the fact that food is hard to get, it's the fact that you can make long trips across the world, or just perform tasks in your base without having to eat all the time. You can just keep a stash of food next to merm king and feed him and yourself every 3 days.
And like I said regarding merm king, he may be something you need to maintain, but the maintenance is super cheap. It's not like you suffer any huge consequences for not feeding him if you're running errands. You just go from half-upgraded-wx stats to wilson-level stats. And if you're playing with other people and are worried about them turning hostile, you can wrap a clever disguise next to merm king along with the food, so they can stuff his face every 3-4 days.
Balance is not thrown out the window with multiplayer. The game is meant to be played multiplayer. That's why deerclops has twice the health he does in DS. Sure, if you add enough players, the game becomes a lot easier, but 2-3 players is what a lot of the bosses are balanced around. The game is not at all balanced for a solo experience, the fact that people are able to solo half of the bosses in the game is cool and all, but the fact is bee queen is not a boss you're meant to fight solo. Nor is fuelweaver. People can do it, and that's awesome, but they would have a fraction of the HP they do if they were in DS.
The ranking I'm giving her is based on what you'd be doing in an average game session with other people. You're not going to be trying to speedrun the game, and you're going to try to make your base presentable and somewhat long term, because that's the direction I've had with every random server I join, and every world I've played with friends. I don't see the point in a tier list if it's going to cater to a more niche playstyle. And I mean no offense by that, mad respect to anyone who has the skills and patience to solo everything in the game. But that's just not how most players do it. The game's called don't starve together.