I use Vivaldi quite a bit next to Opera and Firefox on a Linux desktop. It's hard to really say which I prefer. Vivaldi is definitely more customization than Opera. Vivaldi supports Chrome extensions the same as Opera and over all I find no drawbacks. It supports YouTube videos a lot better than Opera. I'm a music nut and run across many music videos Opera complains about. Just wont play em. Vivaldi and Firefox does though. Not sure why really. I haven't tried Vivaldi's email feature. I'm so use to Thunderbird I don't really need it. Overall, if Opera just stopped working, I wouldn't really miss it that much.
One pet peeve I have with Opera besides refusing to play some Youtube videos, is the page zoom feature. I cannot find a way to add it to the address bar. I have to open a menu to do the zoom then close the menu. If i were able to put it in the address bar like I'm able to with Vivaldi and Firefox, it would be more convenient. Anyway, I find Vivaldi a sold browser, especially if you spend some time to tweak it to your liking. All three browsers mention are very secure and easy to clear data cookies history etc, especially with the help of extensions to automate it all for you. I believe Vivaldi is only closed source with its UI leaving most of what's under the hood open source. Kind of a cross breed.