UI in javascript = still to this day it can take 2-3 seconds when opening new tabs or going fullscreen. Plus they released a statement about "observing" Google adblock decisions instead of going hard in with "we will not deprecate webrequest usability in our fork".

Without source available, it seems like it's targeted for a section of power users.

I have no idea if it would compile and is actually complete, but Vivaldi does do sourcecode tarballs.

https://vivaldi.com/source/

These are the changes made to Chromium. The code for the UI (made with web technologies) is easy to access, but it's under a proprietary license.

What's their business model?

the same business model as all the other free browsers: search referrals