The problem can arise from different water requirements, in that Africans like it hard and high (hardness, PH) and SA/CA like soft water at Ph below 7. This is not the major issue though as most are either already adapted to your home tap water (if you bought locally they were probably maintained in those parameters at the store) or can be.
There are large behavioural difficulties; basically, you can presume that most mbuna are far more aggressive than most SA/CA, despite often being physically smaller.
Also, and maybe more importantly, you manage Mbuna aggression by overstocking which prevents any one or two fish becoming 'King', and you manage SA/CA aggression by allowing sufficient territory for each fish. If you give mbuna too much space they try and own it, if you don't give SA/CA enough space they will go on the warpath and try and clear out the tank a bit. When a fish decides it is king (goes hyperdominant) or that it needs space you have serious problems (often the fish in question may not be able to cash the cheques its ego is writing and winds up dead itself). Remember, they decide who they live with; we just make suggestions.
That is probably in a nutshell why there can often be problems although there can also be difficulties when breeding goes on in the tank and arising from the fact that the fish find it hard to 'read' each others body language when rift lakes are maintained with new world fish. The behaviour and reactions to a given situation are just so different between them.
Equally, although some people have maintained mixed stock with success and you will always hear of the convict which falls in love with an auratus; its just that you wouldn't reccomend the mix to somebody as being likely to work out medium/long term because it is too unpredictable.
Nearly all the fish I can see in your pics are juveniles. Mbuna personality changes radically when they reach adult sizes - requiring all the more careful stocking and management of aggression.