Also, no source available (only a tarball of their chromium fork) so we'll just have to believe them...

In the network analysis by privacyguide or somesuch Vivaldi was among the chattiest browsers.

Tried Vivaldi multiple times. Does not stick.

Some of the issues I saw are

If you use multiple profiles at the same time there is no way to distinguish them on the task bar.

I also couldn’t find a tab handling method I like.

They also don’t seem to release on a regular cadence which makes me wonder how they are doing security updates.

Their UI is not open source.