Dimensional accuracy depends on how well your printer is calibrated. The settings that affect the outside dimensions and inside dimensions (such as holes and other openings) are different, so it’s possible for outer dimensions to be spot on but the inner be off. There are some good calibration tips here: http://www.reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide and here: http://www.reprap.org/wiki/Calibration.
I’ve never gotten mine all the way dialed in and had to sand/file/drill some parts to get everything to fit. Nonetheless, my MPCNC works great and I’m really enjoying it!