Some of that could be due to versioning... Ansible's releases always have breaking changes, so you can get into trouble if your setup is vastly different from when the playbooks were created. I think this is part of the point of a dedicated control machine, but unfortunately the documentation (last I checked) seems to neglect the importance of it.

Along the same lines, there are also .cfg files and roles that can live outside the root directory.

I've been able to get decent reusability out of roles, but that's mostly because I can live with my servers all being deployed the same way (e.g. they all have the same nginx config with minimal customization points and share the same certificates).