Um, wouldn't the moral aspects be in this topic too? Because the moral aspects of the weapons would determine the balance of who would declare war on you or not.
But I think nukes would be much worse than chemical weapons. Nukes kill a lot of people, injure more, and topple hundreds of buildings, and then slowly kill more people via radiation (An interesting addition, I think, would be radiation that could go around and randomly strike a city or a group of cities every so often or so).
The only thing I could see is the chemical weapon as an addition to a missile (We already have biological attacks as a spy mission, and those only kill a certain population, much like the chem would do. Maybe if they changed that to a per turn or something). Not sure the effect of world opinion it would leave, but I imagine it would be pretty bad, though probably not as bad as using a nuke.
sending an armada of tanks to crush enemies can actually contribute to world opinion shifting against you. My first game with Civ 3, I sent a bunch of Samurai to the Persians, got them killed, sent another one, killed again, then sued for peace and tried my hand at the Zulus. Before long, Russia and Babylon had signed an MPP, or military alliance, or something like that. I think a military alliance against me or an MPP actually happened when I attacked Persia.
Granted, trying to move into Babylon after failed invasions into Zululand wasn't the best course of action (Not that it mattered, they never bother to attack you back), and trying to bully everyone didn't exactly help.