Civ is the ultimate game of building and developing and empire.
Why people are fascinated with vaporizing everything with nukes is beyond me.
Nukes are also a cheesy/cheap way out of using real strategy and tactics to win battes gameplay-wise that kind of belittle the combat system (IMO). So I would understand why devs would want to limit nukes. If you put a lot of time and effort into a meaningful combat system you don't want to have it all blown up.
I always stress gameplay > realism but still, softening up your enemies with cannons/ranged whatever then moving in with troops is reasonable. Dropping a nuke to soften up your enemies then mopping up isn't very reasonable because it's just not how it works.
Atomic weapons actually are not a big part of history - they've only been used in war twice, more to end a war than anything. They've mostly been used for threatening, posturing, and "diplomacy." ICBMs have never been used. So saying they must be in the game does not compute.
I guess I'll never understand the fascination with nukes in civ games.
Second - we know what there are no visible diplomatic modifiers.
I could've sworn I read or saw somewhere that you got a big diplo penalty with city states for using nukes, but I could be mistaken - so many previews and such floating around. I kinda doubt the AIs think kindly of it either.
IMO, if they're not going to include the nasty side-effects of using nukes, there should at least be huge diplomatic impact for using them. As with other elements of the game, there should be strategic decisions - use the WMD, but lose some allies, that kind of thing.
Of course, if you're a major warmonger on a roll, you probably don't care about diplomatic penalties.