Aluminum Christmas Trees: Yes, Shakespeare trutherism really does exist, and dates as far back as the turn of the 20th century. Many of the conspiracy theories presented here are ones held by real people, who refer to themselves as "Anti-Stratfordians." And yes, a lot of Oxfordians (Anti-Stratfordians who believe Edward de Vere, Earl of Oxford, wrote Shakespeare's plays) believe the incest thing really happened.
Audience-Alienating Premise: A movie about unironically portraying the single most beloved author in the English language as a murderous buffoonish fraud and one of the most admired English monarchs as having so many bastard children she lost track of them, unknowingly sleeping with her own son and then having another incestuous child. And at an even slightly deeper level of analysis, a movie that requires detailed historical knowledge of the period to understand what’s going on, while also being sloppily researched and screwing up the order of events so that even history buffs will hate it.
Uncertain Audience: The film can be pretty hard to understand without some prior knowledge of Shakespeare and his time period, due to the Anachronic Order of the plot. But the more you know about Shakespeare and Elizabethan England, the more holes you'll see in the film.
Unintentionally Unsympathetic: De Vere is something of a classist asshole and complete snob who is not the most likable of heroes, to say the least.