Those are probably the two most important capabilities for security, so the lack of them definitely means I'd never want to use it for general browsing. I'd much rather deal with weaker keybinds than sacrifice that much on the security and privacy side.
If you want me to try out your browser, you have to support the WebExtension API -- you can support other APIs in addition to that, but WebExtensions are a minimum requirement. I guess Chromium doesn't bundle them, so it's harder for smaller browsers to add the same capabilities?
I'm not sure how Vivaldi and Brave handle it.
Supporting WebExtensions isn't possible without QtWebEngine (the library qutebrowser uses) doing so. That might happen some day, but will probably still take a while.
As for security, see point 8 at https://github.com/qutebrowser/qutebrowser/blob/master/doc/f... for some thoughts on that.