Unless BG adds multi-hex attacks a 100 megaton airburst is really not optimal usage of the weapon. Buttoned up armoured vehicles like MBT's in particular will probably resist this detonation surprisingly well.
Do a ground detonation instead and vaporize the entire hex with a 15-16km wide 100 million kelvin fireball.
Used right this bomb would basically have more or less infinite in all stats as the fireball vaporizes everything that is not a super hardened bunker complex like say Cheyenne mountain. For that however it just needs to detonate like 1km from it and the tremors will make the facility collapse.
For the record this was a planned weapon. The Proton launch vehicle was originally intended as a super-heavy ICBM designed to deliver a non-nerfed AN-602 Tzar Bomba with a yield of 100 megaton of TNT. The AN-602 replaced the uranium tamper with one made of lead reducing the yield significantly but reducing fallout (probably also in part as the reduced yield resulted in the fireball not reaching the ground).
So yes it is entirely possible to argue this weapon "should" be in the game. Improved ICBM guidance however rendered this weapon largely obsolete before it was completed. MIRV likewise makes such colossal bombs less desirable.
Really the Tzar bomba and Proton ICBM was the ultimate expression of what the early ICBM's was all about (that is throwing a huge bomb roughly in the direction of the enemy).
Nuclear weapons is probably not the ideal ionizing radiation based area denial weapon. Large thermonuclear weapons really needs to be used suboptimal for them to do this. Typical detonation altitudes means the fireball does not touch the ground which keeps fallout lower. Likewise the gamma ray pulse is probably not going to be intense enough at ground level to do this.
Smaller weapons will produce significant radiation but with those you need a lot just to block a single hex and even then you probably need to reapply them regularly as most nasty elements are somewhat short-lived.
For area denial with nuclear weapons I would say targeting key infrastructure is probably a better usage of limited assets. Lets say the USSR wanted to prevent the US from reinforcing Europe during a fictive world war 3. Nuking Esbjerg, Cuxhaven, Hamburg, Bremerhaven, Wilhelmshaven and Emden, would make it hard for the US to disembark forces closer than say Amsterdam (my assumption is the Weser is not suitable for this kind of ship movement so no nukes for Bremen). Add Amsterdam as well as Rotterdam to the list and the US might have to use French ports to land enough forces to make a difference.
That said from what I remember one strategy the USSR considered in such a war was basically a rolling bombardment of NATO forces with nuclear weapons in front of the advancing Red Army. The estimate there was that the front-line units would be incapacitated within 6-8 weeks due to radiation poisoning. This however is with the front-line troops regularly getting a new dose of ionizing radiation.
But yes particularly with smaller weapons radiation is relevant. With weapons like Little Boy any area that gets a decent hit with the overpressure shockwave or thermal pulse also gets a nasty dosage of ionizing radiation. For armoured targets in particular it will be the main killer.
In general nuclear weapons should probably affect healthcare. Even if the large thermonuclear weapons does not irradiate people like small ones there will still be a huge number of injured people requiring healthcare and many will probably be invalids for life. There is also burns which large thermonuclear weapons does really, really well.
The damage from nuclear weapons are in many cases not even comparable to normal attacks. It can be far, far worse. If you want to bust a NBC protected vehicle like a M1A2 Abrams you are probably better off trying with some odd world war 2 aircraft like a Sturmovik or Typhoon (not the Eurofighter) than dropping a nuclear weapon on it. According to George Geczy NBC protection reduces attack by 80% for tactical and 90% for strategic nuclear weapons.
That means the best you can get in hard attack for nuclear weapons is 242 for tactical and 180 for strategic nuclear weapons. That is not hard to find. A single B2 Spirit can do that. That is one aircraft not one 12 aircraft unit.
Basically a 25 megaton weapon will hit a NBC protected hard target at around 1/16 the strength of a B-2 Spirit unit. For soft targets it is 1/7.
Also I agree the white flash can be annoying. The game needs an epilepsy mode.
Likewise the AI needs to be better at using them. Nuclear weapons are really the life ensurance of the average tinpot dictator against much stronger enemies (case in point the DPRK). For them using it as a first strike both against enemy infrastructure and military targets really should be possible.
I am actually writing down a proper breakdown of what nuclear weapons does as well as some suggestions as I really am annoyed at how the game represents nuclear weapons hence my firecracker references.
Problem is I keep adding and changing things so I am now at around 5 pages so far and I am not done yet.
Okay this got quite long so to stick a bit with the topic here is an incomplete SM-65D Atlas ICBM. This was the first operational US ICBM and the launch platform that brought John Glenn into space.
It does however carry a mk.3 re-entry vehicle and not the mk.2 carried on the first versions that entered active service.
This is a tiny weapon compared to what the Proton ICBM would have been. Still it is around 100 times the yield of the Hiroshima bomb.
And to complain a bit as a strategic nuclear weapon this ICBM is about as effective as 1-2 B-52 strategic bombers against NBC protected targets (again 1-2 aircraft not 1-2 12 aircraft units)