Yes I know exactly what your talking about and have used many of Chai's rail/bearings.

If your trying them by hand off the machine then it's normal to feel this way. To get a true feeling for them put on flat surface ie:Flat floor and stand on the bearing then move with your weight on and you'll feel they run smooth.

If not smooth when on the machine then chances are the plastic cage may have been damaged when fitting the bearings to rails or one or more balls feel out when fitted.