If raw, they would be much harder to bite into when compared to what a person would expect from a chocolate. Thus, it would be really easy to recognize something was off before you had sunk your teeth in enough to be fully committed. Meaning, a person could very likely just quickly spit it out before getting a good bite.
If cooked, then the softness would make it so you're already all in and you'd get way more sprout all throughout your mouth. Even better would be to cook them so they are super-soft and would immediately become mush in the mouth.
It might be possible to make this taste good. I've made good food with weirder combos. All it needs is a good bridge ingredient that can combine sweet and savory (balsamic vinegar and wine are great ones).
Could also infuse the brussels sprouts with some flavor while cooking them.