I've tried Vivaldi's built-in RSS reader and found it a little rough around the edges. I could see myself switching to it and the built-in email client full time once both of them are feature-complete and a little more polished.
In the meantime, I've been using the Feedbro extension. My RSS needs are pretty basic, so this does everything I need it to. It has options for creating rulesets and tagging posts, but I haven't really dug into those. What I did find useful about it was its feed search function. I don't know where it pulls data from, but it's helped me find feeds for a couple websites that didn't link to or even expose their RSS feed to the browser.
On a side note, I'm not sure I could go back to a standalone RSS reader, mainly because it would need to support some form of content blocking with cosmetic filters enabled. At that point I find it's better to just have everything embedded in the browser, which is why I switched to Vivaldi from Firefox (which used to have Live Bookmarks and other RSS features that got stripped away, unfortunately).