The path measuring tools in mapmyrun (and Google Earth, and for that matter GPS) are accurate. The way people use them to measure courses is not accurate.
None of them is as accurate as a calibrated bicycle wheel for measuring courses, but they are accurate enough that, if used properly to measure the course, no one would complain about the difference they saw from the certified course distance.
Rather than saying mapmyrun is inaccurate, I think it would be better to explain how they used it incorrectly to measure the course.