I've been playing with Home Assistant[0] recently and, as a result, getting exposed to YAML. I don't find it pleasant to work with at all. I'm sure a big part of that is how Home Assistant uses YAML for automation stuff[1] that would probably be better served by a real programming language.
I think a big part of the unpleasantness comes from how non-obvious some things are. For example, why does the first automation example require a "-" in front of "platform" (under "trigger"), but the second doesn't? The comments explain when it's required, but not why? Shouldn't the parser be able to figure it out from the significant whitespace?
I found it so unpleasant that I'm using Node-RED[2] to do anything even vaguely automation related and have relegated Home Assistant to being the UI and communications abstraction layer.
In contrast, XML, while overly verbose, has a more reasonable structure. I haven't played with JSON much, but it also seems pretty reasonable.
[0] https://www.home-assistant.io/