Field maps does honor attribute rules in the smart form on a registered feature layer.
Confirmed through testing, in online workflow.
Pretty Awesome!
Note: Custom error messages written in the attribute rules will not show in Field Maps when rules are triggered by an Insert, or in Field maps 'submit'. Message when constraints are not met is 'Error, cannot submit feature', as of now doesn't look customizable. With training for field staff and hints in the smart form, I think it should be fine.