I don't see how having hard alignment restrictions removes nuance in games. Sure the Cleric of Asmodeus has Lawful Evil on his sheet but that doesn't mean that his character can't have complicated reasons for why he follows Asmodeus. Furthermore, the Cleric may not see his actions or deity as Evil whatsoever.
Maybe he believes that the only way to create an ordered society is by enslaving the unworthy or ensuring that the masses don't have a say in anything. Yeah, that seems pretty Evil to us and likely the party of heroes trying to take him down but that doesn't mean he has to be a BWAHAHA villain. The nuance can still be expressed even through his arguments for why he sees the slavery of others as Good. The players can ultimately call him out on his b~*!!&&%, but that still doesn't mean that the character doesn't have nuance in sense of having character motivations that go beyond just being an a+&~+ who worships a bigger a~$~&@~.