In my experience, you have building UPS on one MDU and General supply on the other. Building UPS will power everything until generators spin up, and if the UPS itself dies then you're still powered from general supply

Did lose one building about 20 years ago when the generator didn't start

But then I assume that any services I have which are marked as three-nines or more have to be provided from multiple buildings to avoid that type of single point of failure. The services that need five-nines also take into account loss of a major city, beyond that there's significant disruption though -- especially with internet provision, as it's unclear what internet would be left in a more widespread loss of infrastructure.