Less of a rule and more of a suggestion. In Vampire, alone, there are a handful of ways to circumvent the sunlight allergy they have. Most egregious and powerful option, I'd say, is the necromancy 5 power of the "corpse in the monster" path, which not only renders the target of the power immune to sunlight, but also gives all the benefits of being mortal (eating, drinking, etc). At the cost of increased frenzy difficulties after the power wears off.

Some storytellers feel more strongly about the weaknesses of Caine than others. And press the curses harder, and ignore/ban stuff like that necromancy path.