They are equally useful and about as well developed.
Comparing Esperanto with Klingon is comparing apples and oranges. No, Klingon is not that developed as Esperanto is. Klingon is for a bunch of enthusiastic Trekkies who wan’t to live their universe. Esperanto has a 136 year old history with a bunch of literature, music and tradition behind it.
No other constructed language has that.
I speak Esperanto for 14 years now. And no, “si” is not a singular “they”. That’s a self-referencing pronoun. And if that usage is used for genderless addressing a person then this is simply incorrect usage, because people don’t know how actually the language works. It’s used in sentences like “li lavis sin” vs. “Li lavis lin”. The first one says “he washes himself” and the second says “he washes him”, the first references the person who executes the action to reference and the second says that the action is done on a different person.
If it comes to Esperanto and genderless usage then there ĝi (it) or ri (they). The first one would be more in accordance with the fundament of the language and the second is a new pronoun which is around since at least the 70s.
No need to misuse si.