I feel the complaint of "too simplistic" can apply to all of the books in this series, and it is unfortunate that this is so, as the underlying idea, the characters, and the setting could have yielded a fabulous set of books.
These books are appealing to those seeking respite from a nasty Western world, certainly. The descriptions of southernmost Africa are enjoyable, as is the notion of honest, good-hearted people in an honest, almost noble, country. But the characterizations are so broadly drawn (excuse a possible pun as to Mma R's girth) as to be somewhat insulting. Yes, the author lived there for a time, but his characters start and end with the idea of "noble savages." I could have told you, after two pages, that a white man was writing about a woman and an African nation. It just doesn't ring true, and in fact, it's rather insulting to the Africans, and to the character of Mma R, to cast them as simple yet honest folk thinking about snacking on a nice boiled pumpkin.
The author doesn't care enough about his writing to keep up with his own details. I think this was just a moneymaker for him, and it's too bad, because we, the readers and the weary Westerners, could have used a more honest and heartfelt effort in portraying the characters and the country.