The GBU-57A/B is a conventional (non-nuclear) device, weighing 14,000kg. (Vs. the W76 warhead on a Trident missile is ~100kg.) Conventional bombs can reliably detonate after taking damage (say, from smashing through 61m of earth & concrete) which would utterly ruin the delicate geometries and mechanisms of a nuclear weapon. And for smaller targets (say, the "deep" bunker under somebody's Presidential Palace), the kinetic energy of 14,000kg smashing through the roof (without detonating) has a very good chance of accomplishing the mission.
They made nuclear artillery shells and bunker busters. This indicates they are not sensitive to shock. Since the impact is a known force & vector, you can pre-deform the mechanism so the impact causes the geometries to be correct, and/or just live with reduced yields from less than ideal geometries.
They do not have public figures for any of the recent nuclear bunker busters, though its likely on the order of 100m based on the depths that various countries have claimed to have made bunkers. The main limiting factor for depth is what speed the materials can handle before vaporizing, how dense you can make the nose and how much mass your delivery system can handle.
Like you have said, nuclear busters are moot as putting 14000kg on someones dining table will still ruin their day without the political issues that nukes cause.
