Labour would be a problem. The typical European holding in Africa during the 1800s relied purely on Forced Labour (due to 2 facts: 1. The natives distrusted wage labour, 2. The Colonies had not enough funds to pay good wages anyway), which of course was very unpopular, and couldn't be overexploited ,would turn the native communities into revolts (well, of course Portugal kind of never cared and fully exploited their natives, and don't get me started on Leopold's Kongo).