This should be less frequent and IMHO, under many circumstance, you are protected by consumer law (citation needed). You also have more freedom when picking the domain name (fuckmicrosoft.co is less than 2$ on GoDaddy, but good luck trying to get fuckmicrosoft.github.io).

Also one issue that I neglected to raise is this can be very unstable. If some scammer/malware use your service (I know that you have a review process, but you still won't know for sure), Google SafeBrowse may mark your whole domain as harmful. I don't know if you have a contingency for this, but you need one in place.

If a registrar dies you could transfer to another one. That's not possible with subdomains since there can only be one issuer

registry, not registrar