You have that with any form of any of these things. They're almost certainly going to be set up so that you get in trouble for claiming that adult content isn't but not for having non-adult content behind the adult content tag.
Then you would be able to avoid legal questions by labeling your whole site as adult content, with the obvious drawback that then your whole site is labeled as adult content even though most of it isn't.
But using ID requirements instead doesn't get you out of that. You'd still need to either identify which content requires someone to provide an ID before they can view it, or ID everyone.
That's an argument for not doing any of these things, but not an argument for having ID requirements instead of content tags.