Until the headline reads "Vivaldi source code released", the browser is irrelevant. They really need to get their act together on this point, there's no excuse for a closed source web browser in 2018.
What do you mean by irrelevant? How does open sourcing it makes it relevant?
If it means privacy, one could just monitor network activity and see if it's making weird connections instead of going through gigantic source repo which most people can't even tell what's going on.
Edge and Safari are barely relevant and my argument applies equally to them. Chrome is just a few patches on top of Chromium, but again my argument aplies - no one should be using given that Chromium is basically the same.
[1] https://help.vivaldi.com/article/is-vivaldi-open-source/