The danger is not just the visible mess. Sewage carries pathogens like E. coli, hepatitis, and rotavirus, and it soaks into drywall, carpet, pad, and subfloor where surface cleaning cannot reach. Those porous materials usually have to be removed and disposed of.
There are also air-quality risks. Sewer gases and, later, mold growth make an unventilated backup unsafe to occupy. Certified crews contain the area, use PPE and air scrubbers, and verify the space is decontaminated before it is reoccupied.
If you have a backup, keep people and pets out of the area, do not run HVAC that could spread contamination, and call a biohazard-trained crew. Document everything for the claim if you carry a backup endorsement.