"What he [Jim] aimed to do was to rehabilitate Heliogabalus in contemporary terms." I think that's the line that best establishes what the biggest issue with Boy Caesar is, which is that Reed put zero effort into trying to portray a historically accurate portrait of the emperor (despite having obviously read scholarship on him, which he is happy to refer to and paraphrase repeatedly) in favour of presenting Elegabalus and 3rd century Rome as no different from circa-2000 London. There is zero nuance to the emperor who goes to underground gay clubs, has an alcoholic husband who dresses in drag, and attributes the Senate and army's disapproval of him to homophobia and "gay-bashing." This could be forgiven perhaps if the author showed an awareness for the extreme anachronisms, but instead the reader is presented with a scene where the protagonist of the modern sections, Jim, has his dissertation supervisor assure him that he's "definitely on the right track" after describing Elegabalus as being the victim of modern idealogical prejudices that literally did not exist in his time. Before reading the book, I had scanned reviews that repeatedly drew issue with anachronisms, but nothing could have prepared me for just how prolific and blatant they are, with there even being two passing mentions of "a deadly virus that seemed to only be affecting the gay community."
Also, despite the Elegabalus sections seeming to be focused on the issue of homophobia, elsewhere—and especially in the modern sections—there are not only hints of Asian fetishization with the character of Masako and blatant misogyny more broadly, but also a clear discomfort with certain subsets of the gay community, as the antagonist Slut is the leader of a self-described cult who appear "trans-human" for their dyed hair, facial piercings, and heavy wearing of leather. This group is immediately established as the villains of the modern sections when Jim witnesses Slut perform sadomasochistic acts at a gathering that he voluntarily attended, which he flees from as soon as it becomes "clear to him that they were about to descend into an orgy."
This book is atrocious, pretentious, and seemingly completely un-self-aware. Don't waste your time.