I used them in the past (many years ago) and was very surprised when my DNS was affected by a cloudflare outage. Turns out (back then) they relied on the cf network for DoS protection against their resolvers[1]. I was surprised to learn that and honestly thought that if I already take a dependency on cloudflare I might as well have them host my zones directly for free.
[1] Not completely sure but I think this was the incident https://blog.dnsimple.com/2020/07/incident-dns-resolution/