Bill makes a good point. Even tho its related to load, the drives alarm setting uses an "encoder count error" to trigger an alarm event. Which triggers an output on the drive. Which triggers an input on the control system. The control system reacts in the manner that it has been programmed to. Take Bill's example above, the error count trigger was most likely set too low. Left alone most folks would set them in that manner. With the same result: a useless machine.
1000 encoder counts per rev is pretty common on steppers. I will use the example of my Acorn project. 1000 encoder counts, set to 250 as the alarm threshold. On a 5mm pitch ballscrew which advances 5mm or .197" per rev the error is set at .197 / 4 = .049". Yup, ~50 thousandths. Set it any less than that and the drive will alarm out under acceleration. Depending on the motor torque and acceleration setting the actual position will often be 20, 30, 40 counts behind the commanded position and the same amount ahead during decel. And that is OK. Here's why:
The drives will apply 100% of available torque once a loss of one native step is detected. Most of these motors have 200 native steps per rev. That's 5 encoder counts. so at 5 encoder counts drive power (amps) is increased until the count matches again. If you set it right, it's like cruise control on your car. Set at 60, come to an uphill section of road it presses on the gas, go down one it lets off. You wouldn't set the cruise to shut the car off if you were off by 1 or 2 mph would you? Besides, most of the time a non encoder equipped stepper looses position to the point where we hear the "ratchet" or slipping sound we have usually lost 3-400 steps or more. A 1/4 inch, an eighth at least. On the Acorn project using a step resolution of >8000 steps per inch and NEMA34 650 ozin steppers the acceleration can be set much higher in this case. Acceleration of 0 to 300 ipm (rapids) in .3 seconds. That's 1000 ipm/sec acceleration. Virtually unachievable with a NEMA23 stepper motor. So even if the bit hit a knot and got behind 249 counts, it would normally be back on position in a quarter second or alarm out.
What you need to remember is that all these awesome features and the extra components and programming cost real money. When the mfgr offers them as an option, most customers buy the lower priced options. When they are standard, customers buy lower priced models. One major mfgr that offers both sells 10 to 1 open loop systems to closed. The consumer CNC market is 100% price, not feature driven. And they publish the specs consumers want. That's the way new CNC buyers want it. That's why I build for experienced users that know what they want and why. They don't run like the consumer grades, for the most part I try to eliminate the keyboard and mouse. And I wont sell a custom machine unless the purchaser agrees to a full day of training. My shop or his. Both cost real money. All the good stuff does.