I think the main issue with renewables is not inherent unreliability but that grid operators do not understand the potential contingencies as well as with conventional generation sources.
SPP has put out some good analyzes of recent max generation events there involving large amounts of wind being unavailable in cold snaps. It turns out no one at SPP bothered to ask wind turbine owners what temperatures they were capable of operating in and some wind operators didn't buy the cold weather package for turbines despite installing them in the Midwest. So generation availability forecasting didn't take into account the real capabilities of generators - a clear failure in operational planning.
This is more of a regulatory issue than a technical one. The machines can operate at -40 C, but someone has to actually spec that functionality and ensure that the associated subcomponents (heaters etc) are properly maintained.
There is a new NERC reliability standard in the drafting stage for this.