I wouldn't have called most of the classes in my MBA program easy or unrigorous. I didn't find just about any extremely challenging as I found some of my undergraduate engineering courses. It probably didn't hurt that I had worked for a few years and probably had significantly better work habits when I got my MBA.

It's also true that the non-quantitatively inclined do find some of a good MBA core curriculum pretty hard. I did some tutoring and in economics, among other things, and I had students who couldn't a simple equation. One once asked me to "explain how graphs work," i.e. basic x-y axis price elasticity, etc. Every few years there seems to be a book written by someone who really struggled with basic math in business school.