Well aside from the realism of the scenario, from a game-play point of view there has to be some kind of downside / negative incentive given to the player, otherwise what's to stop somebody from just spamming Mk 45s at every thing they see?
What I would propose, at least as far as the Campaign goes (even overlooking Soviet retaliatory use of tactical nukes in game)
#1 have a high probability that you just end the campaign right there with "Looks like you kicked off a nuclear war, have fun sailing to Australia and watching everyone die of radiation poisoning....."
#2 If the campaign does continue, another high probability chance that Holy Loch gets nuked (probably along with most of Scotland) in retaliation. What that means for you is, your tender is gone, so you need to do your next mission without resupply, and maybe when you do manage to get a base its something very improvised and can only provide you with say 1-3 torpedoes per-resupply, and no repair capabilities. I don't know how easy it would be to implement logistics into the campaign but I think stuff like that is the only way that you could prevent "nuke abuse."
#3. I think it was shipkiller who was talking on one of the other threads the other day about how bad both american tactical nuclear weapons (MK45 ASTOR and UUM-44 SUBROC). With the Mk45, other than the initial set-gyro (which does have an anti-circle run feature) its command guided only by the wire, has no terminal guidance and must be commanded to detonate as well. What that means for the firing ship is, there's not really going to be any opportunity to get the hell out of dodge. According to one of the nuclear tests in the 50s, minimum-safe distance would be 4500 yards, but this is at PD and I'm not sure what yield. For SUBROC, that distance goes up to 8800 yards but it was also not the most accurate. Both of these weapons are multi-purpose (can engage submarine or surface targets, SUBROC had an airburst feature so I guess it technically could be used for land-attack as well) but the way I see it, the #1 intended use for them was as anti-boomer weapon to be used under circumstances where it was imperative to prevent some type of strategic launch.
#4. While this would mainly be eye-candy, I think it would be pretty amazing to see a Boomer actually performing an SLBM launch which you are trying to stop / minimize.
I feel that there is a place for all of that in this game but it would be no easy task. Steps need to be taken to try to make sure the player respects the situation / weapon ("If I use nukes, BAD things are going to happen.")
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