The cert-manager readme pretty explicitly says at this point under Current status:
This project is not yet ready to be a component in a critical production stack, however it is at a point where it offers comparable features to other projects in the space. If you have a non-critical piece of infrastructure, or are feeling brave, please do try cert-manager and report your experience here in the issue section.
NOTE: currently we provide no guarantees on our API stability. This means there may be breaking changes that will require changes to all Issuer/Certificate resources you have already created. We aim to provide a stable API after a 1.0 release.