> It's open in all of the ways that matter

I disagree greatly here. I'd argue that the engine is the part that matters the least to users, it's the added UI/UX they want to be able to analyze and modify.

Blink won't send my bookmarks and passwords unencrypted to god knows where. The vivaldi UI might. I'd want to see the source for their system. Blink also doesn't have a built-in VPN or remotely togglable experiment system that I'd like to analyze, that's in the closed source part of Vivaldi.

If I want to add features that aren't possible through webextensions, chances are that I need to modify the UI, not the engine, to make it happen.

If I'm a purist, of course I want it all open.