"One area that my controller falls short of the more expensive control boards is it only has an 8 bit counter, compared to 24 or 32 bits of an expensive servo board. This has the effect of only being able to control position error up to +/- 127 encoder or computer pulses. When pushed past these limits the motor slips like a stepper motor does when under load except instead of 1.8 deg. slip there will be a slip in multiples of 127 encoder pulses. This means that the amount the motor slips, or cogs, is a bit bigger step than a stepper depending on the resolution of the encoder mounted on the motor. This motor slipping I speak of is no problem under normal operating conditions, this is because the error should never get close to being that big. If it does it means you are asking to much from the system and need to get bigger motors and amps or slow the thing down. Although my servo controller is not as sophisticated as a full blown motion control card, it is more than the average hobbyist needs, and cost only as much as a stepper system that has no position feedback, if you take into account the fact that stepper motors will cost more."
This is from the website