Land based road mobile missiles are still restricted to the host country's contiguous territory, unless the country is as extensive as Russia and all of it is utilized for hosting missiles, road mobile missiles still represent a well defined threat tube for American ballistic missile defense, unless the Df-41 has a unacknowledged fractional orbit bombardment capability.
SSBN on the other hand, can potentially threaten the target from any direction. Thus an effective SSBN force able to truly patrol in distant oceans represents an exponentially more difficult problem for area ballistic missile defense than land based road mobile ballistic missiles.