Before revision:  These bounds are nonlinear maps of the marginal distribution functions of control and treatment outcomes, and  statistical inference methods for nonlinear maps usually rely on smoothness through a type of differentiability.
After revision:  These bounds are nonlinear maps of the marginal distribution functions of control and treatment outcomes, and not differentiable, which complicates inference procedures, since statistical inference methods for nonlinear maps usually rely on smoothness through a type of differentiability.