Multiline comments are also tricky because of nesting. If you allow nesting,
then this makes it even more difficult to know whether or not you are inside
a comment at any given point, especially in large files. If you don't allow
nesting, then it is easy to end up commenting a section of text that was
already commented and having the inner block interfere with the outer block.
For a data serialization language, they are simply a bad idea in my opinion.
Then again, I would have said the same for program code as I dislike them
for anything other than a file header, even when commenting out long blocks
of code because of the previously cited nesting issues.
As was pointed out, most text editors make it easy to add a comment
character to the beginning of each line, so I don't see much use for
multiline comments any more. Those same text editors also tend to have
code-coloring schemes (as applicable to YAML as a programming language) and
often have trouble with nested comments (were they allowed to be nested),
so, once again, they are problematic.