1. The YAML spec is extremely complex with some things being ambiguous. You might not notice this if your restrict yourself to a small subset of the language. But you will notice it when different YAML libraries and programming languages interpret the same YAML file as different content.

2. Trying to encode logic and control flow in a YAML document is much more difficult than writing that flow in a "real" programming language. Debugging is especially much easier in "real" languages.