I am not saying 6 days is long enough, but if your automation always wait until the last minute to renew certs, you may have more issues to worry about than the CA's availability. If I am going to use a cert with 6 days lifetime I will be renewing it at least once a day.

Yeah, that conflicts with their rate limits, which I hope they'll revise under this scheme.

https://letsencrypt.org/docs/rate-limits/

For the “exact same set of hostnames” (aka. renewals) the rate limit is 5 certificates every 7 days.

So you could do it every other day, if you can make sure there's only one client doing it.

And they're very clear this is a global limit: creating multiple accounts doesn't subvert it.

So you'll need to manage this centrally, if you have multiple hosts sharing a hostname.

If you have multiple hosts the set should not be the same, no? From the linked page the comparison is a set comparison: one host at hosta.example.com and one host at hostb.example.com each with their own cert bot won't conflict.

You never host the same website on two servers?

The servers could share the private key and certificate though