Your understanding is incomplete. Non-nuclear bunker busters are of two types - deep penetration blast types, and daisy cutter types - suck all the oxygen out and atmospheric overpressure shock wave types.
Tactical nukes are a fine substitute - and are more effective in both the "dig 'em out" and "suck 'em out - blow 'em out" modes. In addition the nukes have some beneficial electrical effects that disable nearby electronic equipment that helps destroy the enemy's ability to communicate.
I am not a weapons expert - but read a bunch on the topic. Any corrections - additions are encouraged.
My education is complete.
:O)