I've never heard any expert in nuclear weapons suggest that Russian nukes don't work, especially the ones that have been modernized since the fall of the SU, which is all the ones attached to ICBMs and SLBMs. Until a few years ago, US nuke experts regularly inspected Russian nukes and Russian nuke delivery systems. If Russian nukes don't work, it seems likely that the inspectors would have been able to tell, e.g., through gamma-ray spectrography.
The ICBMs and SLBMs to "deliver" the nukes are more expensive and harder to develop them the nukes themselves, and Russia routinely tests those.
"harder to develop": London, not having had as much money to spend on nukes as the US and USSR had, gets its SLBMs (along with the launch tubes) from the US (whereas they make their own nukes and SLBM-carrying subs) and a few minutes of searching finds no signs of them ever developing an ICBM. (In fact, they might never have had ICBMs: they certainly don't now.)
The ICBMs and SLBMs to "deliver" the nukes are more expensive and harder to develop them the nukes themselves, and Russia routinely tests those.
"harder to develop": London, not having had as much money to spend on nukes as the US and USSR had, gets its SLBMs (along with the launch tubes) from the US (whereas they make their own nukes and SLBM-carrying subs) and a few minutes of searching finds no signs of them ever developing an ICBM. (In fact, they might never have had ICBMs: they certainly don't now.)