Chet, about 10 years ago I finished out a basement for a customer; rec room, bedroom, bath, laundry, daylight windows...the works. 2 weeks later they had almost 4' of sewer backed up into their newly finished basement. Some nearby neighbors had almost 8'; clear up to the joists of the main floor.
When we re-did everything we'd just done, we installed a backflow prevention valve. Previously this area had not had a back-up problem, but a problem with a fast-growing population, heavy rains, and the fact that many homeowners had their sump pumps ilegally discharging into the sewer was the cause.
Under these circumstances, the choice is yours...sewage 4 or 8' deep in your home, or a limited amount of room to flush for a short time until the waters recede.
Thankfully for them at that time, their insurance covered everything, however, today it's unlikely it would; most insurers now have a provision that would call most of this "surface water," since it was originally from heavy rains, even though it came in from the sewer. In a more recent situation similar to this, the homeowner was left to fend for themselves; insurance would not cover the claim. Luckily, their basement was mostly unfinished space, but they lost many antiques & sentimental family items.