I'm surprised by this take. I love YAML for this use case. Easy to write and read by hand, while also being easy to write and read with code in just about every language.
GitHub Actions have a lot of rules, logic and multiple sublanguages in lots of places (e.g. conditions, shell scripts, etc.) YAML is completely superficial, XML would be an improvement due to less whitespace sensitivity alone.
Plus it has exactly enough convenience-feature-related sharp edges to be risky to hand to a newbie, while wearing the dress of something that should be too bog-simple to have that problem. I, too, enjoy languages that arbitrarily decide the Norwegian TLD is actually a Boolean "false."
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.
Language implementations for yaml vary _wildly_.
What does the following parse as:
some_map:
key: value
no: cap
{'some_map': {False: 'cap', 'key': 'value'}}
The other gives me:
{'some_map': {'false': 'cap', 'key': 'value'}}
... and neither gives what a human probably intended, huh?
Most notably it only offers three base types (scalar string, array, object) and moves the work of parsing values to stronger types (such as int8 or boolean) to your codebase where you tend to wrap values parsed from YAML into other types anyway.
Less surprises and headaches, but very niche, unfortunately.