It'd also be a pretty shitty first strike, since you'd be limited to the count of starships scheduled to launch (and likely only ones headed generally in the direction of your target if you really want to mask it) at about the same time. So, probably just one or two, at best. Meanwhile, you'd need at least dozens (of missiles—more warheads) to have any hope of substantially reducing a major nuke-armed opponent's capability to retaliate.

Not remotely worth the complexity of setting up and executing. Maybe worth it against an opponent with extremely limited launch capacity (North Korea?) but that's a pretty niche application.