SPI to Numerous Chips on Different PCBs with FFC Cables?
Low frequency is good, otherwise you would really have to think about signal integrity. But even with low frequency you must ensure clock doesn't ring. So use series resistor near driver and small capacitor near receiver. If the clock rings, slave shift register can shift too many bits. 

Ah, by the way. To make things right, route communication together with grounds. Have the star middle point near the spi master. Otherwise return currents will go their weird ways, and the system will radiate as hell. Of course, it's not good for signal integrity too.