Not true. I house a Jack Dempsey and a Blood Parrot with a Protomelas Taeniolatus and a Sciaenochromis Ahli. There is never any aggression problems or fighting. Of course not anyone can house a jack dempsey and BP with these two africans...each fish is an individual...they can be unpredictable...but saying Africans+Americans=Fail is totally untrue. I know many people who house americans with africans with little to no problems.
Ok, yes every fish has their own personality. But when you mix africans and americans they both have different ph levels to thrive.
Upon the fact that you said many people house them together? ya? what are the chances of them thriving? Did I mention they eat different foods as well?
You sir, are wrong.

They eat different foods? How so? 


.