Well, there is one big problem in brave's implementation of tor in private windows. Tor works by blending you with the crowd, thus making you anonymous. For this, your browser fingerprint should be the same as others. That's why it is recommended to never install extension in tor as they change the fingerprint.
As brave is chromium based, it gives a completely different fingerprint than the official tor browser and so the websites can easily fingerprint you.
As tor browser is firefox-based, firefox could give almost same fingerprint but I still think it's not worth implementing tor mode in firefox. Not everyone needs it and it would make firefox harder to maintain for the developers.
A much better implementation would be to simply fork off the latest-greatest Tor Browser Bundle.
Double bonus points if the thing can be started in Safest Mode, which is What The User Wants in that scenario.
Yes, the user could very well download and run TBB on his own, but most users won't do that. It simply *feels* easier and more accessible if it's already been downloaded, and you can just click "Start a Super Private browsing session" in a menu.