Syncing is usually not that hard. The problem is that you create tiny islands when restoring after blackout and count how much reserve you have if your largest contingency/generator trips in this case. Worst case if you pick too much load after power restoration and suddenly the largest generator trips, you are back to zero. Therefore, companies design their power restoration as small pockets, helps bring all generators online and also helps supporting different island. Synchronizing islands is a bitch but with moderns AVR and frequency regulators they are pretty doable.

Load restoration is a like a total gamble because it’s all done theoretically but rarely any engineers have seen it happen in reality.

Probably not enough fuel in generators. The grid is usually designed to sustain loss of few generators but if the demand power vs supplied power comes dangerously close even worst if the demand power is higher than supply, it can cause voltage to dip which causes generators to run at a higher speed and hence can trip those and results in a cascading effect where the demand cannot be met now by remaining ones and one by one all generators trip.

Coal Generators can take a day to two during warm start , gas ones are quick so depends what’s in their bucket. Power restoration after a grid failure can take a day to a week to complete restoration.