After Hurricane Sandy we seriously considered a water-pressure-powered sump pump (we have 5 sump pumps).
The DOWNSIDE for us were 2-fold:
1) We had to install a new and different plumbing/pipe system to have the water-pressure-powered system - which for us was rather expensive (we live in an apartment building in Manhattan).
2) In the event of a flood, the capacity of the water-pressure-powered pumps was only about 15% to 20% of the regular sump pumps. While obviously 20% of capacity is better than 0% if power goes down, still given the amount of water we faced, we needed the 2000+ gallons per hour of pump capacity - per sump pump (the regular sump pumps).
We have 2000 square feet of downstairs (in a ground floor and basement duplex apartment) - which was filled to the ceiling (almost 8 feet tall) during Hurricane Sandy ... that is 112,000 gallons of water. The water-pressure-powered pumps would not have come close to helping us, even if they were the only pumps operating. Better for us ot go larger capacity - and try to figure out a power back-up solution (still unresolved, unfortunately).
But your situation might be different, and I did really like the concept of the water-pressured-powered sump pumps ... even though they did not fit our needs.