It also is not actually a universal language. The story I recall from a
friend of my fathers who had been very interested in it was that he
gave it up when he met some people who spoke Esperanto, but whose native
language was not in the Indo-European family (I forget the details, but
I think it was a South Pacific language) and he was not able to talk to
them effectively at all. As he described it the limited word choice in
Esperanto meant that too much of the language had to be adopted to fit
the concepts and idioms of the native speaker's own language.
He related this to the "invisible idiot" problem of translation¹ and
said that Esperanto merely amplified that issue such that communication
didn't work at a level to consider it really a language. Now, that is
one person's story and it was close to 50 years ago, but I've heard
similar, if less detailed, comments from other people, though mostly
30-40 years ago when Esperanto seemed slightly more popular than it is
now.
¹ The story goes that early efforts at computerized translation were
done by the US government to try to translate Russian, and the testing
involved taking something in English and translating it ti Russian and
then back to English, The phrase "out of sight, out of mind" came back
as "invisible idiot". Probably not a real event, but running your text
through google translate twice is still a very good idea that would
save a lot of people a lot of embarrassment. We've probably all seem
the various pictures of menus around the world where the English
translation is "translation failed" or "server unavailable" or some
such.
--
'They're the cream!' Rincewind sighed. 'Cohen, they're the cheese.'