For nukes, I'd still go with MAD and them being the last resort button for deterrence.

If you push it, you lose everything automatically. But so does everybody else! So that means you shouldn't use it for random reasons because hey, even if the enemy is dead so are you. But that also means people can't bully you in the corner since the menace is mutual. You nuke me? I'll nuke you!

I think this could and should be used as a deterrent but the option should still be there.

Likewise if you nuke a magical nation which can't (I hope) nuke you back, there would be the fact that for everyone everywhere you will be the big bad guy for ever and ever, expect to be treated like north korea is today, etc. I think its reasonable and that we don't need fancy reasons to make the use of nukes worst, its already the worst case scenario to start with.