Some things feel like obvious oversights, like the lack of “Search for...” when right-clicking on selected text. Also some of the settings pages are broken (like the one with per-site settings), though I assume this is just a quirk of it being a test build.
The only showstopper is the lack of publicly-available source code. Transparency is a dependency of trust. As much as I want to trust Kagi's browser (I certainly have trusted them enough to be paying them every month for their search engine + AI), web browsers are the biggest attack surface on a modern desktop, and I'd be a lot more comfortable with my daily browsing being done with a user agent that has nothing to hide. I don't even necessarily need it to be FOSS (though that'd be preferable); source-available would be enough.