[bluGill]

The important part is that whatever bad happens to mains power, the backup systems detect it and properly take over. Bad frequency or power out are both bad situations that the backups need to handle. You need to have confidence that your systems will handle both. If you only pull power and never test bad frequency, one phase missing, brown outs, over voltage, (list not complete because I don't know everything) then you have not done your job. If engineering determines that your test also covers another bad scenario then I'm fine with not testing it.

[brudgers]

Electric utilities provide appropriately higher levels of service and communication to hospitals. Hence there are protocols insuring hospitals don’t lose power in the way you are imagining. The people involved are serious competent professionals with extensive relevant experience, training, and education focused on safety and in touch with the specific facts of actual hospitals and utility grids.

[bluGill]

There isn't much that they can do about storms knocking down wires, squirrels chewing a wire and knocking out a phase (other than taking down all phases). Which is why hospitals have several backup systems.