If you're the sort of person who gets into arguments about gender-neutral language, you've probably heard (or said?) something like this before:
Beware of ambiguity: Because the pronoun “they” can be used to refer to persons in both the singular and plural as well as to things in the plural, ambiguity may arise as to what “they” refers to in some contexts.
This warning comes from a Government of Canada guide to gender-inclusive language that I came across recently.
On its face this seems like a reasonable concern; after all, the guide goes on to give some concrete examples of sentences where the singular "they" is ambiguous:
Ambiguous "they": The medical examiner may issue the certificate to the applicant if they have met the requirements set out in section 27.
Is "they" here the examiner or the applicant?
Ambiguous “their”: The Commissioner or a person acting under their direction in carrying out their duties under the Act must not disclose any information relating to an investigation.
Does the second "their" refer to the duties of the Commissioner, or the duties of the person?
I agree that these sentences are ambiguous. But don't let that distract you from the fact that these are not examples of the problem that they're supposed to be an example of.
Specifically, these are not examples of confusion caused by the fact that "they" can be used for both singular and plural subjects. On the contrary, these examples deal only with singular referents: the medical examiner, applicant, Commissioner, and "a person" are all singular.
If we replace "they" with the old-fashioned male-by-default "he," the sentences remain equally ambiguous:
Ambiguous "he": The medical examiner may issue the certificate to the applicant if he has met the requirements set out in section 27.
Ambiguous “his”: The Commissioner or a person acting under his direction in carrying out his duties under the Act must not disclose any information relating to an investigation.
Oh no! It turns out that gendered pronouns like "he" are confusing and should be avoided due to their potential for ambigiuity!
All Pronouns Can Be Ambiguous
The singular "they" is no more prone to ambiguity than "he," "she," "we," or "you." Every example I've ever seen of a sentence with an ambiguous "they" can easily be tweaked slightly to give a parallel example of some other pronoun being ambiguous, and "you" even has the same supposed problem of failing to distinguish between singular and plural.
The reason we're not awash with ambiguous pronoun referents is simply that fluent speakers are pretty good at recognizing when a sentence could become ambiguous and avoiding it preemptively, often without even realizing. If there is any increased incidence of ambiguity associated with singular "they" -- and, to be clear, I haven't seen any empirical evidence that there is -- it is probably due to unfamiliarity or overthinking causing people to make mistakes that they would normally easily avoid, rather than any inherent grammatical problem with the word.