Let me preface this by saying that I actually like Trent Horn: he is a pretty solid Catholic apologist (see his 2017 The Case for Catholicism [Ignatius Press]), he has done great pro-life work, and we have had some friendly exchanges on his facebook page (e.g., this exchange after his debate on the Protestant canon). With that being said, he continues to be pretty weak on "Mormonism," and in a recent video, even plugged his flawed 20 Answers: Mormonism booklet by Catholic Answers:
Do Mormons get their own planet in heaven?
On his booklet, see:
Answering Trent Horn on "Mormonism"
To give credit where credit is due, however, to Trent, he does not argue that LDS theology is "polytheistic" but "henotheistic" (I prefer the term "Kingship Monotheism," but gotta give him props for being much more accurate than many other critics of the Church).
If/when Horn does more studies into The Church of Jesus Christ of Latter-day Saints, I do hope he sends drafts of any chapters/books to informed LDS (I would be willing to review such and I am sure I could wrangle Dan Peterson, Blake Ostler, and other friends who are "heavy hitters" in LDS apologetics/scholarship to do the same, as there is no good volume out there critiquing "Mormonism" from a Roman Catholic perspective [think a RC equivalent of the 2002 The New Mormon Challenge, or to use a secular example, New Approaches to the Book of Mormon: Explorations in Critical Methodology).
Robert Boylan
ScripturalMormonismATGmailDOTcom