I am not sure why everyone seems to think inventing their own domain-specific language is a good thing. It isn't.
But also I prefer using open source projects from communities and companies who are based around open source. Google do contribute a lot to open source but they're primarily an ad company who want your data. So consider using buildah a type of boycott. ;)
I use buildah privately whenever I need to quickly make an image to test something, it's so simple to use in bash. It's also very simple to add to existing images while you're testing it. So it was just natural to continue using it in CI pipelines.
I only ever used one command for kaniko so I'll admit I know nothing of it, and that was 2 years ago. But buildah feels a lot more accessible because I learned all the different shell commands that correspond to the Containerfile format.
I started using buildah to build container images in CI jobs because then the job can run in an unprivileged container, unlike docker.
At the time I only knew of kaniko that could do this but I preferred buildah for a number of reasons.
Buildah is really awesome at multi stage builds because you can use it from bash.