I switched to Opera some years ago because it was fast, seemed to have a low memory footprint, and it was possible to have lots of tabs open (a vice of mine, I admit, but also very useful!). The tab scrolling function that was added (or maybe I added from the settings?) not too long ago has now disappeared. So when I have, say, 60 tabs open, they're all squashed together so tightly that it's impossible to select one of them. And the disappearance of tab scrolling means I can't even see what they are anyway. Before "One", I not infrequently had around 1,000 tabs open with no similar problems. Madness, you say... ? Yes, I agree. But the reason it tends to happen for me is because of the nature of my work. At least in the old days I could register the fact that I had an absurd number of tabs open, then go through them and close a whole bunch. That, too, is now basically impossible. (Incidentally, if Opera would fix these issues, and also add a "page" where you can see all of your tabs in an array, like in Pale Moon, it would be the perfect browser fromm my point of view.)
But also - and worse - is the fact that the browser now freezes (for example, whenn I'm trying to scroll up or down on a page) and crashes quite often, with far, far fewer tabs open than the previous version managed without grumbling at all.
Clearly, "One" has stuff going on under the hood that I, and most users, will be completely unaware of that's eating up memory capacity and bringing the whole thing, in my case, shuddering to a halt. Maybe that's all essential security stuff, or something else that's very important - but I have my doubts.
Anyway, I'm moving back to Firefox or Pale Moon until they restore what, for me, was Operas vital USP.