There is nothing especially confusing about it. Any command can be confusing if you don't read its description.
As for criteria, no such general criteria have been stated explicitly, AFAIK. A judgment is made by the developers who distribute Emacs.
It is typically not so much about inherent confusion regarding what the command does or how it works as it is about confusion after the fact: confusion about what happened.
And even more importantly, whether inadvertent use of the command might cause changes that are difficult or impossible to reverse (undo).
In Dired, my guess is that the reason
a is disabled by default is that someone who uses it, perhaps inadvertently, loses the current Dired state, with no warning and no confirmation required. The buffer is killed.
If you had gone to a lot of trouble to omit certain files and subdirs from the Dired display, to insert certain subdirs, to mark certain files or subdirs, to use particular
ls switches, etc., then it would take some effort (and some memory) for you to restore the same Dired state. On the other hand, if the Dired state is the default state then you can simply use
C-x d to visit the directory again.