Caveat: these are my anecdotal clinical observations over years of experience, not necessarily based on a deep, sturdy, objectively sampled and analyzed data set.
That being said, I find that learners on the autistic spectrum often do struggle with the open-ended nature of the essay, especially with certain aspects (not necessarily every aspect in every individual), including idea generation, organization, elaboration, and the personal narrative qualities of the prompt. GT students (and 2e GT/ASD even more so) sometimes get hung up on the absolute factual accuracy of the essay.
I did find this research reference (it's a meta-analysis, so you can poke through the citations, too):
https://www.ncbi.nlm.nih.gov/pubmed/29164435
