Russia can barely maintain the gear they have that costs orders of magnitude less than a hypersonic missile / nuclear torpedo. Whats the chances their hypersonic missiles / nuclear torpedos even work?.
I wouldn't make any decisions on the thought that they won't work. Even if 90% of their currently deployed nukes don't work, that's still 160 deployed nukes that work, which is enough (not for outright extinction, but still). And 90% would be a high estimate given that Russia is genuinely very capable at rocket tech.