Even with portability issues, Vivaldi is way better than any other browser out there. Tabs on the side. Visual tabs. Tab stacking. Search field. Much more options than in other browsers (if you discount things like about:config and chrome://flags). All that as native features without extensions. The old Opera feel is back.
Unfortunately, Vivaldi indeed has portability issues. In addition to leaving registry traces behind, it loses my extensions and passwords when I switch computers. (Extension preferences remain though, so I don't need to reconfigure the extensions once I have reinstalled them. Still, it's a pain to reinstall them constantly.)
When it comes to losing passwords, I think Vivaldi uses the same method of saving passwords as Chrome, with the same limitations. See portableapps.com/apps/internet/google_chrome_portable:
Passwords Not Saved Between PCs By Default: Google Chrome stores passwords in such a way that they are encrypted in a way tied to current PC. While the passwords are not kept or left behind on the PC itself, they won't be retrievable when you move to a new PC.