I use Vivaldi as my main browser because of one thing mainly: it doesn't change. There were no significant UI redesigns since 1.0. The changes that they did make were all in service of new features that wouldn't work in the previous design, and the scope of such changes is always as limited as possible.

It's also like the old Opera which I'm very much used to. It's also not dumbed down like everything these days; there are more settings than I'll ever need.

Though another thing that is also exactly like the old Opera is unfortunately their attitude towards quality control.