In Africa this process seems not to work as well. According to one 2007 study of 90 developing countries, Africa is the only region where urbanisation is not correlated with poverty reduction. The World Bank says that African cities “cannot be characterised as economically dense, connected, and liveable. Instead, they are crowded, disconnected, and costly.”
I say the one big problem is premature deindustrialization:
What ties them [African cities] together, and sets them apart from cities elsewhere in the world, according to the Brookings Institution, an American think-tank, is that urbanisation has not been driven by increasing agricultural productivity or by industrialisation. Instead, African cities are centres of consumption, where the rents extracted from natural resources are spent by the rich. This means that they have grown while failing to install the infrastructure that makes cities elsewhere work.
That is from The Economist, the article is interesting throughout.