The F-RAM device VDD ramp rate is traditionally specified as us/V to give a system level perspective of the minimum time requirement for the device power supply (VDD in case of FRAM) to ramp up and ramp down. This unit of measurement was also adopted to avoid any confusion with op-amp slew rate V/us. Most integrated circuits set a minimum finite time for its power supply to rise to its min operating voltage so that device can correctly set its internal band gap, logic and peripheral circuits and configuration for a successful boot up. In most cases (including FRAMs) power up ramp should be monotonic; hence, putting a minimum time for VDD to rise also doesn't provide a complete picture on the device power up requirements. Hence us/V unit was chosen to make it more clear requirement in device datasheets.
As you pointed out correctly - minimum us/V ensures power supply regulator should not ramp up or ramp down too quickly. In case VDD ramp is too fast, it must be slowed down within device specification. Cypress's FRAMs do not specify max ramp rate (max us/V), thet means device guarantees to operate for all possible slow voltage ramp rates that can be observed in any system.
I hope this clarifies.