I've used Thorium in the past and thought it was decent. But given Google's updates to the chromium project I've moved away from chromium based browsers.

I thought the whole downside of chrome permeated anything chromium and any other subset of it.

I’ve used Thorium (not as my main browser) and I like it. Decent privacy features, performance does feel better.

Some major downsides though:

1. It is not frequently updated to the latest Chromium patch; there have been times where Thorium has lagged three major versions behind. And just forget about getting patches that fix major security vulnerabilities until the next major update.
2. The browser is heavily opinionated, and while that has resulted in a browser with a half-useable version of the Chrome refresh, see this issue and it’s clear the focus is not on privacy.

If you want a browser that’s more focused on privacy and don’t care about the eye-candy that Thorium provides, the Cromite browser is only doing security + privacy patches, has toggles for more permissions, has V8 disabled by default, allows for automatic clearing of history, allows you to change the default referrer policy, has more chrome://flags, and actually gets updates frequently to the latest patch.

Thorium's entire focus is on performance. As another commenter has noted, that means no security updates, and no privacy features.

I wouldn't recommend it for daily use, but if you are playing a browser based game it's worth testing out. I used to play krunker.io and I tested it to see if I could get more FPS (FPS equaled faster movement speed back then), but I didn't see any major performance improvements over the major krunker clients or Microsoft Edge (other most performant browser).