At 2024 NET Camp, people had questions about sewage pumps, and whether they flood houses with sewage in a disaster or a power outage. What’s a sewage pump? It’s a pit, where sewage and grey water collect. At the bottom there’s an electric pump that turns on when the pit reaches a certain level, to pump the sewage and grey water out to the sewer line. (Note: A sewage pump is not a sump pump.)
Sewage pumps are required for houses with bathrooms below the level of the main sewer line.
Houses above the sewer line discharge sewage by gravity.
Here’s a diagram I found:

So the answer depends on the situation, and on how full the sewage pit is to start with. In a major pipe-breaking disaster, sewage will NOT back up into the house, because there will be no incoming water to fill the toilet and add to the sewage in the pit. Sewage will be the least of your problems.
In a power outage, the pump won’t turn on, and that could be a problem. If sewage and grey water continue to be delivered to the pit, without a functioning pump, the pit contents could back up into the house, or onto the ground outside.
In the January 2024 ice storm and prolonged power outage, Multnomah NET’s Team Leader, Tom Boeker, went door to door in his low-lying, sewage-pump-equipped condominium complex, asking neighbors to minimize water use to avoid overfilling the pit. There was no sewage spill, and a lot of neighbors got to know Tom.