I find the mechanical difficulty—the actual process of integrating sexual progression into the plot—is consistently tricky. It's not just about adding more scenes; it's ensuring each one moves the characters or the central conflict forward, or it starts to feel like padding. I've read so many books where the tension gets resolved through a lust-driven power-up, and it completely deflates the stakes. Suddenly, the villain is defeated not by wit or growth, but by a supernatural orgasm. It feels cheap.
A subtler challenge is maintaining character integrity. When a character's 'level' is tied to their sexual experiences, it can accidentally reduce their complexity to a mere tally of encounters. Their motivation for intimacy might become purely transactional—'I need to sleep with this person to gain this ability'—which erodes the possibility for genuine, messy emotional connection. The best examples I've come across, like some arcs in 'Kushiel's Dart', manage to weave desire, political maneuathing, and personal sacrifice together so that the sexual element is a catalyst for deeper change, not the change itself.
Balance is the other huge one. Too little focus on the lust mechanic, and why include it at all? Too much, and the non-sexual plot can atrophy. Readers pick up these novels for a blend, and if the world-building or external conflict gets neglected, the whole thing can feel one-dimensional. It's a pacing nightmare, honestly. You're constantly juggling the reader's expectation for steam with the need to actually tell a story.