Shall we throw some numbers around?
Seems like it might need a new rule or two: would a tactical nuke have a "Total Annihilation" area? I'm not sure how many tac nukes are aimed at direct fire/contact detonation, I'd guess few are, air burst for most with delayed detonation for weapons intended to damage underground targets. Is there a current day tactical nuke that's similar to the "Bunker Buster" LGB? How much fallout would it create if it detonated underground? Will the weapon survive the impact to detonate underground?
The warheads would need a radiation value, rules to implement the effect of are already in place. A rule for how much fallout and where it drifted would be needed along with how long it would last.
Following on from the earlier thread where nukes in space were postulated to be less effective as they have no atmosphere to create a shock wave, would there be call to vary the effects based on atmospheric density? I guess in theory this could also be applied to any explosive (are there rules for thermobaric and FAE warheads?)
How far from the blast do you need to be to avoid blindness from the flash? How long will it last?
What else is there?
Edited to add, OK, a little google fu reveals there are nuclear bunker busters so scratch that part of the conversation...
Oh, and no, I don't envisage running a game with nukes but then I don't run games with fusion gun armed grav tanks either and don't remember the last time I ran into one as a player...