In the US, at least, nukes are considered part of the "base load" supply. It is not the reactor that is the problem, it is the boiler and the piping and the turbines that do not like to be thermally cycled. And those parts are pretty much the same in a coal or nuclear plant.

Solar and wind can be curtailed, but are not counted as "dispatchable" power because they cannot be turned on at any time (solar, only in the day; wind, only if it is windy). A gas-turbine can be started from cold in a few minutes.

I think there are some terminology problems, but that is how they are categorized in the US industry.