The Horseman very much wants to be a dark, atmospheric treatise on the nature of obsession and love, and to present a tragic tale of a talented yet homely antagonist rejected by the more potent evil he adores. Unfortunately, the adventure also serves as the info-dump for the metaplot in preparation for the final adventure in the series, The Darklord, and since this happens right up front, the sense of fatigue and irritation that this will likely install in your players will color the entire adventure.
Begin with the knowledge that the events of the previous adventures were irrelevant -- in The Raven, the party races against a 'ticking clock' to rescue the seer Sybil Rasia, yet regardless of their success, Sybil begins this adventure with the party, and even manages to bear the plot-significant object from The Artifact if, for some reason, the party either hasn't played the adventure or has somehow misplaced the MacGuffin.
The party is informed that the adventure's antagonist is on the way to deliver the final piece of the main villain's plan to her, yet has no way to impact this -- by the time the party mounts up and rides to the rescue, Omou (the antagonist) had already made his delivery and been rejected by the object of his own obsession, leading him to once again subject the poor, hapless citizens of Orasnou to dark doom. If it seems like 'endanger the citizens of Orasnou' is an overdone trope by the thirteenth adventure in the series, well, your eyes don't lie to you -- of the five Season Four adventures largely set in or involving Orasnou, four of them (including this one) feature an assault on the village that the PCs must thwart. Only the very first module, The Beast, doesn't include a direct threat to the village.
Another problem with the module is its attempt to raise the stakes by attempting to convince the DM to use 'Breathless Pacing', by which the module designer means 'don't let the PCs take even a short rest if you can help it'. In general, an adventure where short-rest focused classes like warlock and fighter are not necessarily advantaged over long-rest focused classes is a good thing every so often to serve as a change-of-pace, but this adventure doubles down on the challenge by adding arbitrary damage between encounters and basically seems to want to drain the PCs of all their useful powers before finally confronting them with the main antagonist of the piece -- who is honestly powerful enough to deal with a nearly fully-rested party if run intelligently.
A competent adventure, yet despite its attempts to provoke bigger emotions (one section is actually titled "The Last Stand of the Greenhall Elves"), it simply comes across as tired and waiting for the end.