As far as I know, the big damage from Nuclear Weapons planetside is the massive blastwave that can pretty much scour the earth, with radiation and thermal damage bringing up the rear.
But in space there is no atmosphere to create a huge concussive and scouring blast wave, which means a nuclear weapon would have to rely on its all-directional thermal and radiation to do damage… but is that enough to actually be usful as a weapon in space, considering ships in space would be designed to handle radiation and extreme thermals due to the lack of any insulative atmosphere?
I know a lot of this might be supposition based on imaginary future tech and assumptions made about materials science and starship creation, but surely at least some rough guess could be made with regards to a thernonuclear detonation without the focusing effects of an atmosphere?
The electromagnetic pulse may not cause physical destruction, but it would likely disable any spacecraft in the blast. Which could result in death and destruction when the passengers can’t breathe or get warmth and the craft loses control.
Wouldn’t a spacecraft have a Faraday cage anyway, to protect the electronics from stellar winds?
That might reduce the impact of a given EMP.