Every service should strive to provide perfect availability, the reality is that it’s not possible to be perfect. Mistakes happen, it’s how you deal with them that is important.
Successfully dealing with availability issues requires planning, and when dealing with a client-server solution, it requires both parties to make improvements.
In the context of ACME today the large majority of clients have no failover or logic. This means if the enrollment fails due to connectivity issues to the specified CA, the CA has an outage, or the CA suspends operations for one reason or another the enrollment will fail.
This is a problem for any service protected with TLS, which is basically every service, that wants to have a highly available service. One of the ways services deal with this is to proactively acquire two certificates for every endpoint, this is viable, but I would argue this is not the right solution.
The right solution is to not rely on a single CA and instead failover between many CAs that are capable of servicing your needs. This way if any single CA fails you are fine, you can just keep chugging along.
That is not sufficient to address this risk though. Not all certificates have the same level of device trust. Sometimes clients make bad assumptions and trust specific CA hierarchies assuming these configurations are static, even when they are not.
To help mitigate this behavior clients should implement a round-robin or random CA selection logic so that subsequent renewals will hit different CAs. This will force clients to make sure they work with any of your chosen CAs. This way you won’t find yourself breaking apps that make those bad assumptions when your CA fails.
Caddy Server already implements both of these strategies as I understand it but every ACME client should be doing the same thing.