I use them as my upstream DNS provider for pihole. I've only run into a few cases where I've had to switch to something else because their anti-malware DNS results was overzealous. I don't recall the specific cases.
I was using Quad9 until they had infra issues where latency skyrocketed from ~40ms to an unstable ~250ms... soon after I noticed that the routing was changed to another country with a latency of stable ~70ms.
A DNS resolver in another country breaks several things for me such as Wi-Fi Calling and certain CDNs that do not support EDNS Client Subnet, so I'm using Cloudflare Gateway DNS (DoH) for the time being.