Still probably impossible to fire enough sneaky missiles to disable enough ICBM silos. I think they are an interesting development, but I don't think they are meant as first strike weapons against another major nuclear, at least not without excepting a full lobby back.

They are terrifying against a state without a "nuclear umbrella". A few of these sneaky nukes could take out any ability to strike back conventionally. (Thinking of Sweden. Not that it longer could strike back anyway since about 1992.)

As long as nuclear subs are a thing there is no possibility to take out the other side's ability to attack first.

Think about how quickly drone technology will evolve. The amount of explosives needed to destroy a sub is fairly small I presume. That means it should be feasible to mass produce underwater drones to cover coastal area, such that missiles launched from subs should be outside the zone that's too close to defend. I think AI / drone technology will completely change the military landscape fairly soon, as producing them will be cheaper than defending against them.