Especially if they're MIRVed. And ESPECIALLY if they're highly accurate and capable of taking out a hardened silo.

They are extremely expensive, and delicate, vulnerable to first strikes, so they have little use as a deterrent. For deterrence, you spend the same money and build many robust and simple missiles, and disperse them in as many hardened silos as possible, and target your enemy's cities so you'll have the ability to punish him if he attacks first, with a survivable and numerous weapon you can hold in reserve until you're sure you have a real war in progress, not an accidental one.

I realized the US missile capability was a first strike force when I saw them going to the trouble of MIRVing submarine-launched missiles, what is supposedly our ultimate deterrent weapon. We were designing them to lead the assault on the USSR, not respond to their attack. Of course, the Russkies new damn well what we were up to.

It should come as no surprise, Nuclear doctrine in the 80s was dominated by the lunatic theories of Hermann Kahn, who felt that publicly brandishing and communicting a threat of a first-strike capability would complicate Russian war planning and force them to go broke trying to cover every possible contingency. (Hey, does that sound familiar?...look up Star Wars and SRBMs in Europe). Of course, it put everything on a hair trigger basis and made an accidental or panicky war much more possible. And the true capability and potential war strategies we were preparing for were carefully hidden by the press, so it was not clear to the public just how destabilizing our policy was.

The Russians thought we were crazy and aggressive war-mongers, and reacted appropriately. But as an American working in the Defense Industry, I knew the real reason.
It was actually quite benign. We weren't belligerent, just greedy. The military-industrial complex was so keen on developing and selling new and expensive weapon systems that whether or not they prevented war, or even were capable of winning one, was simply irrelevant. Victory was defined by getting the contract.