But that's not available on Android or Windows (the WebKit port exists there, but no good UI frontend as far I know)... which are the two big platforms. (because Apple doesn't care enough about those I guess?)
It does replicate a lot of buggy behaviour in Safari so from what I can tell you can be reasonably sure that your website works in Safari if it works in WebKit, but that's the only use I've found for it so far.
As for a good UI on Windows, I think you should be able to compile GNOME's WebKit browser for Windows. It'll look terribly out of place like any GNOME application in a different environment, but I think it should work well enough to use?
If you used it in the past from the Debian repos, they didn't use to do security or feature update servicing for WebKit in their repositories. That's no longer a thing nowadays.
You can use both the Technology Preview at https://webkit.org/downloads/ or check if the version is recent on your distribution however.
What? One of the main points of the project is to build a browser with a whole bunch of tinfoil like tracker blocking and anti fingerprinting measures. They wrote their own built-in adblocker with Rust so it won't get fucked by Manifest v3 since it's part of the browser. They're the only Chromium browser that does CNAME uncloaking, something that uBlock Origin can only do on Firefox.
Containers are unique to Firefox, Chromium browsers handle that via separate user profiles.
Plus, Google controls Chromium and Firefox is literally the only alternative now. They push changes that are against or in in ignorance of of web standards because they can. Downstream browsers like brave,edge and opera basically have to accept whatever Google says. This isn't good for the web at all. Frankly there should be an anti-trust suit against Google for this because it is very anti-competitive.