I've been a member here for 3 1/2 years, and to be honest I've never ever come across anyone mentioning problems with the E9x's A/C reliability. We have quite a few members from extremely humid asian countries who seem to do fine, on the other hand we also have members living in places that are stupid hot (but a dry heat) like Arizona and no complaints there either. So like you said, it could be the combination of hot and humid that's doing it, beyond that I'm at a loss
Edit: Do you get South African or German-built BMWs there? I wonder if they source different brand evaporators between the two.