I was a die-hard Opera user when it ran Presto - I tried the Chrome version for a while, & I have Vivaldi installed so I can periodically open it & try it out for a while, but absolutely everything since Opera 12, Vivaldi included, has paled in comparison.

Opera 12 was instantaneous in everything it did, even with a session with 100s of tabs open (without auto-unloading them in the background like modern browsers do) & thousands of local emails in M2. The instant history navigation in particular is something no modern browser has even attempted to copy, Vivaldi included (likely because it's a core Chromium functionality that would be difficult to override).

There's just so many tiny details of its UX that were slick & seamless & have been lost. Little things that seem minor but were huge on aggregate like text selection of linkified text - it simply does not work in Gecko or Blink browsers but somehow Presto did it with ease. The page you're leaving remaining fully responsive during navigation to facilitate change-of-mind on mis-clicks, etc. Millions of tiny UX details like this just made the whole daily browsing experience so painless.

I bet it is a great browser, but I did not get the same feel as the old opera at the time when I tried, too many features missing back then.

Moreover, not using chromium-based browsers is a kind of matter of principle for me. Chromium has been a monopoly for very long, which gives google too much power on how people may experience the web. This was made especially apparent with the manifest 2 -> 3 transition, but it should have been seen as a concern imo since a good while back.