The problem I have is, I need a configuration language that is human readable, familiar to developers without teaching (anybody who knows JS, or to some extent Python and PHP), is also machine writable, and supports comments. The only language that supports this is JSON+comments (next contenders would be INI (!) and then XML). JSON5 just helps by giving it a name, a consistent implementation, and packages for many languages. I've rolled my own JSON+comments (or "static subset of Python / JS literals") multiple times, and am glad there is a community standard now.
The trailing comma is really an issue, because it works in JS, and it is an arbitrary restriction in JSON. If you learn JSON on the job (and think of it as "JavaScript Object Notation" i.e. JS literals) then you will run into this, and it causes unneccessary frustration.
Since every JSON5 package I know produces valid plain JSON, compatibility is never an issue.