I wouldn't recommend Quad9 (9.9.9.9) for personal use either.
During some very brief testing I ran into performance issues like 1500 ms response times and UDP timeouts.
Also, via the Global Cyber Alliance, there is quite a bit of partnership with various government entities:
https://www.globalcyberalliance.org/community-partners.html#partner-industri...
This leaves many unanswered questions. What criteria is used to define "malicious" traffic? Who gets to add domains to the blacklist? Etc.