Using it with Tor is almost certainly not a good idea because it changes your own behavior from other Tor users thus compromising your anonymity (and the Tor folks are not in favour of PrivacyPass, because they think the solution is that CloudFlare shouldn't be putting the reCAPTCHA in the way in the first place). And that's assuming that the cryptography is actually solid and there is no way to distinguish between different PrivacyPass users. Tor has decades worth of research put into it -- what level of scrutiny does PrivacyPass have? How many people actually use it and how many have tried to break it?
> When 80% of traffic from an IP is malicious and the other 20% is regular traffic, but both sources look like the same traffic (impersonating browser headers, sometimes running headless chromium), what else can you do? Cookies and stateful cookie-like objects, such as privacy pass.