Nukes can solve a lot of problems, and create an entire bunch of them in the process due to their inherent nature and the whole geopolitical theater. In the end it creates more issues than it solves.
On the topic of hypotheticals and practical approach however, if the US ran out of MOPs, to be efficient, to some extent, the tactical nukes would still be required to tip a bunker busting ordinance.
The US has an entire range of bunker-busting devices, not many can reach as deep as the MOP however. So, there is the issue of delivering the payload at a sufficient depth.
Sure, a nuke develops significantly more power than regular conventional explosives, however, depending on where it is used, the blast and overall destructive effects would be heavily mitigated by what it is being surrounded with: granite.
Enough underground nuclear tests have been conducted to anticipate what a strike would look like. The mountain wouldn't collapse upon itself, there would be cave-ins (either, and, or, local or peripheral) and lots of heavy/significant seismic activities.
So, from a physical perspective, it could damage/destroy deeply buried facilities, just like it could also not do anything (depending on how deep they are, how reinforced they are, etc...).
Another point to consider is the fallout. Underground detonations produce little to no leaks of radioactive materials due to the chamber collapsing upon itself and thus sealing itself shut. There can be some release, however, it has happened (see the French Beryl test at In Eker site in Algeria).
Could there be an upward release of dust? Maybe.
How about downward release towards tunnels? Maybe as well. Seismic activities would possibly damage or weaken the tunnels (again, depending on how deep and reinforced they are), thus possibly allowing for radioactive materials to leak in.
And then there is the whole can of worm surrounding the fact it would be the third time a nuke is used in anger, and how it would open a number of doors and impale the US on the international stage.
Aside from these considerations, another question to contemplate is: why is it being talked about
now? And only now?
In the early stage of the war against Iran and its nuclear program, sites like Fordow were the main topic of conversation and focus of air-strikes.
In fact, Fordow was considered as the most critical objective which also represented the greatest challenge the US could face. The way it was built and the configuration of the infrastructure made it virtually impervious to anything but a specific countermeasure designed by the US. In other words: to deal with it there was one way, and only that one single way; and nothing else short of ground operation.
Take it as an assault on Mount Suribachi, if you will.
Well, now that Fordow is ancient history, Pickaxe is the new contender.
It would be rather strange that the site got left untouched and avoided practical military treatments from the US and Israel if it was, indeed, such a threat.
It would also be rather strange if Iran had somehow managed to sneak the necessary equipment to that location, while being actively slapped days in and days out, without attracting attention.
It would be strange if Iran also had the necessary resource to conduct such a project, first without anybody noticing it before hand (before the strikes on Fordow, for instance), or, second, while the US and Israel were busy conducting strikes against the IRGC.
If these elements are not to be treated as strange and are to be taken as factual, then it would imply Iran nuclear program is much more advanced and their available resources much more concerning.
A rather fitting analogy would be a colony of ants.
To deal with ants you don't just tackle the anthill, you have to go after the ants and the queen. And to do so you usually target the ants who will be used to kill the queen.
Stomping the surface won't solve anything.
Fordow was an anthill. It got dealt with. Or so we thought.
Until Pickaxe appeared and proved that dealing with Fordow didn't end up with dealing with the problem.
If, hypothetically, Pickaxe is also dealt with, who knows if in a few months we won't see another nuclear site hiding another mountain?
What that Pickaxe problem suggests is that the only way to deal with the Iranian military nuclear program isn't to go after said program, but to deal with the IRGC. The IRGC is the queen.
