I think there is more than one mechanism through which harmala-containing plants cause nausea:
Having had to take mebendazole as a kid many times for pinworm infections, there was no nausea whatsoever from those parasites dying off.
- Effects of the harmalas on the nervous system, as @northape pointed out.
- Other potentially nausea-inducing substances, such as tannins or high amounts of fructose (the latter, found in caapi brews).
- Interactions with food eaten in the last day.
- Psychological reactions.
Having had to take mebendazole as a kid many times for pinworm infections, there was no nausea whatsoever from those parasites dying off.