Hey Bill,
welcome.
This debate has been had a couple of times in our community because people like using Quad9 and the other Quads. However, they are sometimes causing technical problems and they are not uncontroversial.
The technical problems simple are filtering some resource records which simply will break DNSSEC. You might argue that people should not land on those filtered websites anyways and this does the job. However, there is no reason that I can trust you doing “the right thing™”. A man in the middle could hijack your responses, too.
So for purists, Quad9 breaks DNSSEC.
Regarding the organisation that is running it: People have different beliefs. DNS traffic is very sensitive traffic. Quad9 in this instance will basically get a full browser history. That in the wrong hands can be dangerous or simply not wanted by some people. It simply is privacy.
So you will have people who will trust the organisation behind it and you will have people who don’t.
Being backed by various large corporations that do not have a good track record in dealing with people’s data properly.
The blog article states this:
For privacy reasons, it is best to stay away from big resolvers run by profit-oriented companies, such as Google, Cloudflare, Quad9 and others. They do not sell cookies, and they do not provide service for free.
I think that holds true. IBM is a large profit-oriented company and so is PCH. We all have to have our salary paid for. And the article then raises that those services are not being provided because people love to do so. I am sure it is okay to assume that there is some (monetary) benefit in there for them.