I like to see some evidence of something happening, otherwise everything ends up being linked to everything else without any basis.

You see there is plenty evidence of Brave planting spyware and needless cryptoware into a web browser. Seeing it is upto your bias towards usage of that browser in exchange for a few useless bucks.

https://nakedsecurity.sophos.com/2020/02/27/brave-beats-other-browsers-in-privacy-study/

The study you linked gives off weird vibes because it does not try to make it clear that Firefox telemetry can be turned off with a couple toggles. Instead it says it turns the identifier persistence "silently on by default".

The other issues it mentions are usage of Google's Safebrowsing API and autocomplete search engine predictions being on my default for Firefox. These are credible depending on your threat model. If it is a lighter one, this might not be a concern. And these are a toggle away from being turned off. Users being this technically illiterate will have more problems on the internet than mere autocomplete prediction pings.

Quoted paper by Leith also mentions:

The issue for many of these browsers seems to be not so much what they’re doing, as the fact that they do it by default, leaving non-techie or unaware users open to more information gathering. From Leith’s paper:

In summary, Chrome, Firefox and Safari can all be configured to be much more private but this requires user knowledge (since intrusive settings are silently enabled) and active intervention to adjust settings.

Brave's autocomplete fiasco is what set off most of the privacy community to get away from it. Also, there are a fair bunch of Brave shills on the internet doing rounds these daysz going from reddit to 4Chan to HN and wherever easy to target privacy communities exist online.

Brave comes off as a grifter in the privacy browser industry because it leaks IP via WebRTC a lot, leaks via so called Tor mode, has hardcoded cryptomining features which is junkware, and has done a fair share of "oops just a bug we will fix it" things for a browser that claims to be gung ho about privacy, yet uses Chromium as its base and has its own adblocker giving you a false sense of adblocking (while inserting its own ads), unlike how uBlock Origin does it.