Micro finance did not work in certain parts of South Africa. That's not surprising. Neither this, nor anything else is a "panacea" (cure-all) for various financial problems.
Micro finance apparently works in SOME parts of the world (India, Bangladesh, etc.), where the ethos and institutional framework make it viable. It may work better with women (who have less access to other credit and are more susceptible to peer pressure), than with men.
Practically by definition, microfinance is not highly scalable. It works, if at all, "locally," where local lenders know and understand local entrepreneurs. That's why it doesn't lend itself well to large institutional activity from e.g., mutual funds.
The fact that it works SOMEWHERE is very much to its credit, and probably what won it the Nobel Prize. In so doing, it made the world's financial "space" somewhat more "complete" than it would otherwise be.