IMO to be enable to build nukes, one should have to build a specific building first — maybe a land-based nuke silo or a new specific building, a bit like the factory HQ (the idea being that this building processes nuclear materials while the launcher builds the missile).
Alternatively, nuke-subs (plus the Sera. battleship) could be given an upgrade option to enable nuke-launches, if there's desire to make them useful without nukes.
Burst fire would be most useful if it allows multiple targets (essentially rapid-fire TML).
As-is, nuke-subs are (a) a sneaky, slightly cheaper way to build nukes on maps like Roanoke and (b) a cheaper way to try overloading SMD. IMO the SMD missiles are still too expensive. But, getting it right might also require adding cheaper conventional-warhead "decoy" missiles to the SML.
Wait, did anyone notice that although the nuke-sub's nuke does less damage in the inner ring (22k vs 70k) it actually does more in the outer ring (3k vs 500) for the same radii (30 inner, 40 outer)? Also, the nuke silo has a big death-weapon where the nuke sub doesn't.