There is no problem with YAML, just like there was no problem with XML at all. The problem is with the software devs who use it for the wrong thing. I just wonder why JSON doesn't get all the hate, while it is a terrible format also: it is not streamable (YAML is), it doesn't have comments... and non-standard workarounds are used for these features. Norway better than YAML, in my opinion.
There numerous problems with YAML. It is underdefined, ambiguous, fragile, and is just impossible to write a portable parser for. JSON is naive, lacks almost everything. But is supereasy to parse and read. As funny as it sounds, XML is overspecified. Parsing is possible and predictable but supporting the full standard (all of them) is so much work that nobody does it anyway. So, given the choice above, i'd go for nai…
XML just looks nasty, but IMO not worse than JSON. JSON is not easy to read, because brackets suck for deep nesting. XML nesting is 100x easier to read, because the closing tags tell you exactly what is closing.