The most common attack I see on any new pronoun/s is, but they’re invented / not grammatically correct / not standardized / not widespread.
First of all, this argument is based on two things: the person’s personal discomfort with the pronoun/s of another, and society at large as the gatekeeper to acceptability.
To deal with the first, it doesn’t matter what you think about another person’s pronoun/s. It doesn’t matter if the pronoun/s are yt or xie or ae or ou–still that person’s. Still legitimate. You still ought to use them. To not do is to further binarism and cissexism, and to put your minor comfort over another person’s identity and lived experience. It takes a truly astonishing degree of self-supremacy, arrogance, and colonialism to believe that another’s gender is up for debate–or up to being defined by you, for that matter. Nobody holds the keys to being ‘truly’ or ‘really’ or ‘actually’ any gender–or lack of gender.
Second of all, who gives a damn if society says that pronoun ‘x’ isn’t standardized? Who on earth cares? (For the record, singular they is correct, and it is not always a bad pronoun as well.) And, why, one might wonder, aren’t ae and ze and ne and thon already correct? Why is it that English doesn’t have more pronouns?
My pronouns are invented–so what? Do you think that she fell out of the sky? No–it was invented, just like every other pronoun and every other word and almost every foundation of the entire human world. Being invented means that something exists, not that it’s been given the mark of Cain.
To accept invented pronouns and new words is to accept that English is not inerrent–it is to acknowledge that the commonplace, the usual, the average, the blank-slate-not-really does not fit all. There are other worlds beyond reach of the English without modern inventions–do you really want to cut yourself off from them?