It's been nearly 2 weeks since I moved over to #vivaldi and honestly I'm impressed (coming from #firefox and #chrome), it syncs nicely between all my devices (even iOS). Two issues off the top of my head:
1. It would be nice if the desktop tab stacks worked more like Chrome's tab groups. Lack of colours/labels and the constant expanding and contracting is annoying.
2. Why are bookmarks on Android so hard? I just want to bookmark a page, not manage my bookmarks every time. And even when I click 'add current page' it doesn't do anything so I have to manually copy the URL.
I think they're literally the only things.