There have been a lot of people claiming recently that singular "they" is too confusing, especially for English as a second language speakers. However, I haven't seen anyone point to any studies, and as this Duolingo forum argues, many things about all languages are confusing, but that's not a reason to avoid using or teaching them.
Imagine a Duolingo German course without genders and articles.
Imagine a Duolingo Spanish course that decided not to include "el agua" (feminine word with a masculine article), "la mano" (feminine but with a masculine ending), or the days of the week (lunes was a plural declension of the singular lune. But in Spanish, "lunes" is both singular and plural), and any of Spanish's flood of irregular verbs.
Imagine an English course without the pronoun "you", which started plural and became both singular and plural and yet is always paired with plural verbs. [...]
[...] It's incorrect grammar. (What wasn't considered "incorrect" at some point or other?) It's controversial. (What part of language hasn't been considered "controversial"?) Language changes. It changes regularly. Pronouns are not exempt from change (though this isn't a change for Singular They, which has been around at least since 1300. Rather, it is a resurgence.)
The whole original post is worth reading as I think it makes its point intelligently.
So, coming back to the question, has anyone actually studied singular "they" to see if it's confusing for either native or foreign speakers? Is it something that foreign learners simply can't grasp, or is there no basis to these claims?