Okay so I see this question and my mind immediately goes to the godawful execution in so many indie scribbles that use 'taboo' as window dressing. Genuine tension? It's less about the shock factor of the situation and more about the constant, grating friction between raw want and the fear of losing everything. I've read some where the central conflict is the terror of being found out, obviously, but the really powerful stuff explores how the characters internalize the taboo—like that story where a protagonist is torn between pure adoration for their partner and this deep-seated belief that they are, at their core, a monster for feeling it.
That internal war is the engine. External threats are just the pressure cooker. Without that self-loathing, that gnawing doubt that maybe society is right, it's just a regular romance with extra steps. I put down way more books than I finish because they ignore that psychological messiness in favor of cheap, manufactured drama. The tension dies when the characters don't genuinely struggle with the morality of their own desires.
A specific thing I look for is whether the narrative lets them off the hook too easily. If the resolution is just 'and then everyone accepted it,' I feel cheated. The real emotional payout is in the hard-won, fragile peace they make with themselves, not with the world outside.