I think people are seriously underestimating Brawler.
With genuine respect
@Charlaquin has there ever been a subclass where someone didn't say "I think people are seriously underestimating [subclass]?" Because I've even seen it said of Purple Dragon Knight and er... the crummy elemental Monk one.
I don't think we are. I think we're skeptical for good reason. It's a class reliant on two things which are unproven:
1) Weapon masteries - you are right that that factors in, but I give high odds Weapon masteries get ULTRA NERFED before they go live. Like nerfed to the point where most of them might as well not exist and just add needless extra book-keeping. You seem to be believing they're going to go live in or very close their current. I do not believe that.
If they do, then yes, things look up a fair bit for the Brawler and for martials in general in combat - in fact martials might even get
slightly OP in combat (honestly fine and fair given the decades for which that wasn't true!).
2) Reliance on non-existent magical items. This is a big one. WotC have failed to deliver on most of their claims/promises/intentions so far, I'd suggest, with this series of UAs specifically (less so in general - but DND Next had the same pattern).
They keep saying "We want to do X" and then they do Y, instead and X is never mentioned again. And there's no press to ask them "Yo what happened to X?" because they don't talk to the press, by and large, outside of emergencies or pet people like Todd Kenreck (who don't get me wrong, seems like a genuinely great dude, but who has never, ever, to my knowledge, asked a hard question, or even question that wasn't kind of obviously prepared for, of a WotC employee). Genuinely ENworld and Linda Codega are the only people positioned to do so even, but I don't think ENworld has the access (sadly) and Linda's primary beat is indie RPGs, not hold WotC to account - that sort of started by accident.
If, and it really is an if, these magic items appear, even then, because this is 5E, not 3E (where you could make them) or 4E (where you could basically suggest them to the DM), you're reliant 100% on the DM being nice enough to give you a specific novel item. Something my experience is that a lot of D&D DMs are loathe to do, sad to say.
(Weirdly I often end loaded down with huge numbers of magic items in 5E games I've played in, but like, none I wanted and often ones that don't help or further the theme of my class at all. I still get good use out of them where I can, but like, I worry.)
Whether a character gets an appropriate magic item for combat is equally dependent on the DM whether the item in question is a "magic weapon" or an item "that enhance{s} Unarmed Strikes and Improvised Weapons".
No. Sorry. Not true.
There's a huge difference between a class reliant a singular, specific item, like "Gloves of Brawling" or whatever, and a class that can benefit from
any magical weapon, potentially. Certainly any martial one.
The latter is going to be drastically more common, and absolutely
@fluffybunbunkittens is 100% right to point out DM goodwill is going to who gets what in 5E.
So I'm sorry that equivocation is just not valid.