It's less about YAML itself than the MS yaml-based API for interacting with build-servers. It's just so hard to check and test and debug.

This so much this. Vscode has a very good syntax check github actions yaml so it's not yaml that's the problem.

It's the workflow for developing pipelines that's the problem. If I had something I could run locally - even in a debug dry-run only form that would go a long way to debugging job dependencies, etc. Testing failure cases flow conditional logic in the expected manner etc.