Time. I wouldn't trust it while it's new. I'd develop trust in it over time as I've observed the results of other people using and examining it.
> Would you ever pay for browser security ($3-5/month)?
I don't rent software, so I wouldn't pay a recurring fee. A one-time fee isn't out of the question, though.
> Is Manifest V3's limitations (30k rules, webRequest restrictions) a dealbreaker even for security-focused extensions?
Pretty much, in that I wouldn't be using a browser with that limitation in the first place.
(I develop Privacy Badger.) There are significant benefits to adding PB or uBO to a browser that doesn't already ship with a real built-in ad blocker. While PB and uBO work well together and you may want to use both for various reasons, I wouldn't say you need both. Either one is enough by itself for most people.
>HTTPS Everywhere
HTTPS Everywhere has been deprecated and eventually removed from extension stores a few years ago: https://www.eff.org/deeplinks/2021/09/https-actually-everywh...
>Phishing detection
Why isn't what's built into browsers enough?
>Cookie auto-delete
Why bother when blocking trackers and ads?
>Pop-up blocking
Is that the same as the various "annoyances" ad blocker lists?