^ I am pretty sure that the positive reports do not imply safety. That is, I suspect that a bunch of people have used MDMA with mild doses of harmala without having any adverse effects.
Having said that, the main reason behind the caution is that both MDMA and Mescaline trigger a release of serotonin, which will be increased by the use of MAO-A inhibitors. The relative rate this increase is taking place is totally unknown, which means that while some may get away with the combination, there is a very real danger of causing a serotonin syndrome (too much serotonin - leading to seizures) and/or hypertensive crisis (too much serotonin, leading to major increases in blood pressure and heart rate - leading to seizures).
Again, the reason for the caution is that we simply do not know the threshold that a wonderful experience may end up a life-threatening nightmare.