I used to be an engineer for GM and had to investigate situations that sound very familiar. In particular the customer would complain that while braking the vehicle would go over a rough surface such as a rumble strip section and experience longer stoping distances. This was due to the sudden change in wheel rotation caused when the wheel may momentarily loose contact with the ground or the bumps caused the wheel to change rotationsl speed enough to trigger ABS operation. Since the primary function of ABS is to prevent a spin out the pressure to the rear brakes is reduced and the customer will feel a brief but noticeable change in stopping power. While this is not a desireable situation the spin out that used to occur in cars such as the Citation caused so much bad publicity that ABS systems became a highly desired feature in most cars. I have investigated several situations where police officers complained that the ABS system caused them to crash. In all cases it was the normal safe operation of the ABS that caused the officer to believe his brakes were not performing when in fact they were working as designed. I have taken a Prius out and tested it under the same conditions that I know will cause some customers to complain and sure enough the brakes behaved in a manner similar to what I investigated in GM cars and trucks. I latter worked for Hyundai and noticed similar braking characteristics in their ABS systems also. While it is easy to say they think there is a problem with a vehicle it is much more difficult to say exactly what is causing the problem. When you sell millions of vehicles if there is a design problem you should get millions of complaints or at least thousands that you can duplicate. If the condition cannot be readily duplicated in specific vehicle it is not correct to conclude that there is a problem. Untill the condition can be duplicated readily it is a situation that needs more investigation to determine if there is a problem.