Feature-wise, Vivaldi is great indeed. But every time I actually try to switch to it as a primary for any prolonged amount of time, I inevitably run into major perf issues and/or deal-breaking bugs. In fact, my most recent attempt was just about a week ago, and vertical tabs w/grouping had numerous visual glitches for me, with tab buttons occasionally becoming taller than the screen, white rectangles that are supposed to go around tab groups having half the width that the tabs inside do etc. I "solved" this by using the Windows pane instead, and all was fine, until the browser just started randomly lagging to the point where everything slowed to the crawl.
And yes, I did report it to their bug tracker - which is private, by the way, so there's no way to know who else is having those problems, or how work on them is proceeding. I will say that the bugs I reported on the last attempt are fixed, but it's little consolation when new equally bad bugs show up in their place. And as for perf issues, there are numerous active threads on Vivaldi forums about them.
I'd love to use Vivaldi as my primary, esp. now that they also have an iOS offering, so tab & history sync across all of my devices is possible. But it really needs to be more stable.