I check in on Vivaldi every few months to see how it's coming along, and there are some things that I like. However, I think they're a bit TOO fixated on Opera as it was 15 years ago.
I keep waiting for bookmark sync, which today is a fundamental browser feature rather than a nice-to-have. But the Vivaldi guys insist on pouring resources into an integrated desktop email client instead, because that was a signature feature of Opera back in the day. However, it's probably no exaggeration to say that most users haven't used a desktop email client in over a decade. So why is such a retro throwback feature prioritized so high?
Another example is right-clicking a link in Vivaldi, to open it in a new tab. Every other major browser opens the link in a background tab, keeping the focus in the tab you're currently on. However, Vivaldi automatically jumps the focus over to that tab right away... because that was default Opera behavior 15 years ago. This is frequently brought up in Vivaldi forums, and the response from the devs is simply to grumble that old-school Opera was best and all other browsers today have bad defaults.
I'm a huge Opera fan, and have been for years. However, the thing is... I just don't WANT a re-implementation of Opera from 15 years ago. I'm sorry, but desktop email is dead for most of us. I'm sorry, but Chrome has acclimated us to some slightly different default behaviors, and at this point those are the defaults that I expect and want other browsers to mirror.
You want to enable a whole new level of customization for power users? That sounds AWESOME! You want to provide some security and an alternative path forward, for Opera fans who worry about that company's ownership changes? Great! But please don't be so beholden to the Opera of 2002 that you don't make an offering well-suited for 2017.