I've seen some solar guy explaining that the inverters that create alternating current from solar (and wind, although that's more complicated) are very good at /following/ the phase and frequency of the grid, but in that, they /amplify/ disturbances. Turbines (hydro, nuclear, gas etc.) on the other hand have a lot of inertia, /dampening/ disturbances.
So, »stability« is not just about reliability, but also about inertia — »rotating mass«, as some call it.