I've been there. Not YAML specifically, but basically just configuration (XML, JSON, properties, ...) for some proprietary systems without any good documentation or support available. "It's easy, just do/insert X", half a year and dozens of meetings and experts later, it was indeed not just X. Meanwhile I could've build everything myself from scratch or with common open-source solutions.

I mean...building a data centre / PaaS with YAML is pretty cool

We used to have to shove servers in to racks ! Kids these days :D