It's used in production in the past 3 companies I've worked for (a movie company, a gaming company, and a computer hardware company). In all cases it's used to templetize resources in a CI pipeline -- although others have told me they use it as a very, very slim Helm replacement.
In my current case, I'm working with a Google product that requires a Rego policy to be embedded into a YAML for Kubernetes, so this allows embedding it (see in the Readme the last example).
Currently, it cannot output multiple files in one run. How would you envision that? Would you say you would prefer passing a folder and any file inside of it is templetized or rather you would see multiple "-f" flags? Let me know, happy to work on it, since it's also a need I've been told from other parties too!