With the mob I fly for Captains and First Oficers are required fly to the same standards in the simulator, i.e. the environmental limits of the aeroplane, the only difference being that the Captains are always PF for the lo-vis stuff and RTOs.
There are however limits for First Officers for line operations which are more restrictive and which I believe are due to liability and insurance considerations.
In a case of a Captain suffering incapacitation then the First Officer is fully capable of operating the aeroplane to it's limits.
Been there done that when a Captain I was flying with suffered a kidney stone attack en-route and I had to complete the approach and landing in conditions which were right on charted minima but well below normal First Officer approach minima.
Regards,
BH.