From the POV of countries that do not have tons of such bombs is much better, also bombs exploding inside bunkers or submarines should make less damage then exploding on areas where it was computed it would make the most damage.
But IMO this will be used to scare the nuclear weapons owners, they will need to invest a lot into shielding all their bombs or reducing the number of such bombs, probably at that time when this tech is possible we will have antimater bombs
Nope. Unless you take out all the thousands of nukes at the same instant, you are guaranteed to get at least some of them sent your way. Doesn’t take many to wipe out a large part of your population.
The path of radioactivity through the missile facility would point at the facility with the beam, because the showers of induced particles would be along the same axis.
You then nuke that country out of existence, as is standard policy if governments fear an attack on their strategic capability.
You could also, eg, track the facility ingesting that incredible amount of power — whose signature would be basically impossible to hide from satellites.
But IMO this will be used to scare the nuclear weapons owners, they will need to invest a lot into shielding all their bombs or reducing the number of such bombs, probably at that time when this tech is possible we will have antimater bombs