Using any chromium fork is supporting chromium as the default browser, and choosing gecko-based browsers means you throw your hat in the ring to show websites that not just chromium browsers visit them. Using a chromium fork also means you accept that google will dictate standards less harmful than web integrity. While it's not the end of the world, the last time this happened IE9 happened. Using Firefox means google can't simply decide to implement any spec they want and that spec becomes true because all user agents have it (exactly what is at risk of happening here. Vivaldi may make nice blog posts but if this ends up happening they'll be happy to implement it else risk losing their entire user base)

The would have been an argument to be made about performance but nowadays blink and gecko are pretty much at the same performance level.