Hydrogen vs. Battery Electric Cars

Given a watt of power, you can get farther with a BEV than you can with a hydrogen vehicle. Therefore the scenario you pose - if all cars were BEVs and they all plugged in at once  the grid would fail - would be even worse with hydrogen. But only if the hydrogen is made with electricity.

And there's the issue. Most hydrogen is not made from electricity splitting water molecules.

So that's an efficiency I'd like to see. Would it be more efficient to generate electricity from natural gas and then charge BEVs, or is it more efficient to create hydrogen from natural gas and fuel hydrogen vehicles. I suspect it is not but in the context of your scenario it is the more valid comparison.