The problem is when a malicious package is produced, either because a flaw was introduced in the code, or because a dev machine was compromised, or _when the CI machine sad compromised_; the malicious package will be signed as if it were legit.

One middle point between automated and manual signing is, as usual, key rotation: have the signing keys expire in a short duration of time (say 2 weeks) and manually push them every week, so that the window of attack is as small as possible.