Foreign inspectors might warn the Russians about the risk of theft, accidents and so on. But would they warn them if they noticed that the weapons were degraded or nonfunctional, unbeknownst to the Russians?
Also, the Soviet Union as well as Russia do have a culture of corruption, plan-fulfillment-only-on-paper, diversion of public funds and organized inefficiency on the job. E.g. from the current Ukraine-Russian war, there are reports (to be taken with a grain of salt of course, but there are dozens of confirmed, similar stories from Soviet times) of food shipments containing just canned water instead of canned meat/vegetables because somebody embezzled the money and shipped water to cover it up. I can't really decide either way: maybe all this is even easier, given the secrecy surrounding everything military and nuclear. Yet maybe it is harder, given the increased scrutiny around all things nuclear.