During the recent partial outage of Cloudflare's network[1] multiple other DNS providers stopped working, because they were all using Cloudflare's DNS Firewall[2].
The affected services included:
– DigitalOcean DNS
– Linode DNS
– DNSimple DNS
It would be good to know any DNS providers with anti-DDoS protection that won't go down with Cloudflare's network.
[1] https://news.ycombinator.com/item?id=31820635
[2] https://www.cloudflare.com/dns/dns-firewall/
If you're concerned about availability, you'd never run your servers on something that depends on one company. Even if you use a service that does depend on one company, you'd also want to run your own DNS services somewhere else. If you don't, then you don't really care about availability.