The ß-carbolines are not magic-bullet MAO-A inhibitors.
In addition to inhibition of MAO-A at ordinary dosing and MAO-B at high dosing, ordinary doses modulate the activity of the CYP metabolic enzymes.
Harm(al)ine induces CYP1A2, 2C19, and 3A4 activity and inhibits CYP2B6, 2D6 and 2E1 activity.
I forget if its harmine/harmaline or the other P. harmala alks that inhibits CYP1A1 (a pro-carcinogen activating CYP).
This is relevant because after alkaloids get into the blood they are primarily metabolized by the CYP system, so it should be expected that they will effect phenethylamines, tryptamines, acetaminophen, other stuff, etc.
To muddy the waters, some drugs are really pro-drugs. Mescaline seems to be in this class. If I remember right, mescaline can be metabolized primarily by 3A4 or 2D6. Many anecdotal reports relate a potentiation of mescaline with harm(al)ine, those reports seem more consistent than potentiation-by-grapefruit reports. Perhaps CYP3A4 is the pro-drug activating metabolic pathway and 2D6 is the pro-drug destroying pathway, for instance.
Anyway, point is ß-carbolines have nuanced pharmacological 'personalities', to think of them only as RIMAs is overly simplistic.