They are added for a few reasons.
The first is for shield heavy power armor (particularly shield focused, and especially on modded armor), and firing them at extreme range, which does permit survival (particularly of the 0.1t), and deals with behemoths very effectively, and can be a compliment to bigger weapons.
The second is for vehicles (particularly modded vehicles), which can have long range machine guns, high health, equipment grids, etc. and can allow use of the rounds as a secondary weapon (with e.g. rockets as a primary, and lasers as a close quarters defense).
I've found them generally relatively survivable, though far more risky than other weapons, provided you consistently fire them at long range, and single-shot them.
The most dangerous weapons added are the shotgun shells, which are just there for completeness, and because it's funny sometimes to fire a huge barrage of small nukes.
I generally try and give the player lots of choices, and let them work out which ones are bad choices, particularly as with mods (like additional power armor, sniper rifles, or vehicle weapons) some of these options become way more useful than they would have.
I generally play with aircraft enabled, and the nuclear rounds are often pretty good in those (though still wildly dangerous).
