I agree - YAML is not suitable for complex cases that people use it in, like Terraform and Home Assistant. My pet peeve is a YAML config in a situation that really calls for more abstraction, like functions and variables. I'd like to see more use of the class of configuration languages that support that stuff, like Dhall, Cue, and Nickel.

There is another gotcha which is that YAML has more room for ambiguity than, say, JSON. YAML has a lot of ways to say true and false, and it's implicit quoting is a bit complex. So some values that you expect to be strings might be interpreted as something els.