It depends on what time of the year it is. During Winter Peak on the East Coast (ISO-NE/PJM/NYISO) or Summer Peak in the ERCOT/SPP regions, you might not have a huge amount of reserves left. The shoulder months would be much nicer, but that is when many generators go on outage for repairs, so you have other problems.

One of the biggest issues is that traditional resources (coal, nuclear, gas) can run for long periods of time and in many places have a pretty reliable fuel supply. Renewables are great, but you could be generating lots of power one minute and then it's all gone the next hour. This causes a lot of stress on traditional resources which need to be ready to counteract this. Coal plants and gas plants were never designed for that either and it's causing issues.