Although he starts that same paragraph:Small quantities of khat, Catha edulis, along with harmel, are not at all unpleasant.
so there's every reason to be cautious.Some of these combinations are dangerous, particularly combining harmel (or any other MAOI) with amphetamines or amphetamine-like compounds. The sympathomimetic effects of such compounds, with their usual metabolic degradation inhibited, could be unpleasant or life-threatening.