It's not that certificates themselves are insecure themselves, it's that the workflows (as the parent points out) are awful. We might still add some automation around that (and I think I saw some competitor tooling out there if you're committed to that path) but I personally feel like it's an answer to the wrong question.
Whut? This is literally the opposite.
With CA certs you can create short-lived certificates, so you can easily grant access to a system for a short time.
However, it provides you an additional layer of protection, because it does not need to be on the critical path for every SSH connection. My CA is a Nitrokey HSM, for example. I issue myself temporary certs that are valid only for 6 hours for ephemeral private keys.