What is the difference between 4 way hydraulic spool valves designed for double acting cylinders and those designed for motors? What be the drawbacks of controlling a motor with a cylinder valve?

Of the approximately 10 spool center conditions available from most valve manufacturers, NONE are considered Motor Or Cylinder Centers.

Some training books call a "Float Center" ("P" Blocked, "A" and "B" to "T") a "Motor Spool" since it allows the motor to coast to a stop without damage that can be seen when using an All Ports Blocked Center Condition. However an All Ports Blocked Center Condition used with Cross Port Relief Valves can quickly stop a motor without damage and little overrun revolutions.

All center conditions may give advantages in certain applications and when applied correctly can usually make an actuator operate in a smooth controlled manner.