I really doubt that's as general of an occurrence as you make it up to be. There's a particular government process I can only do on Edge (no other browser works, chromium or not). For a certain login process in a different branch of government I can use Vivaldi or Firefox, but not Edge. I don't think you can single out a browser for this kind of thing.

Do you have examples of websites that don't work? Both because I'm curious and also so the devs can look into it?

I have issues with sites on Vivaldi but it's never due to Vivaldi itself. It always ends up being that case that either uBlock Origin or Vivaldi's own built-in ad/privacy blocker ended up blocking some javascript library that the site needs.

ProTip: Try to do your thing in Guest Mode. It will almost certainly work there.

I've never seen or heard of this before. Perhaps it is a user error.

Not sure what country you lived in, but having lived in different European countries, I never found this issue.

Have been a Vivaldi user for many years.