There have never been so many luxury brands dotting the Miami skyline. As you walk through Brickell, one of the city's most upmarket areas, construction hoardings bearing the logos of Cipriani, Dolce & Gabbana, Mercedes-Benz and Mandarin Oriental line the seafront. The migration of billionaires from New York and California, drawn by the tax advantages offered by Florida, is fuelling this frenzy for ultra-luxury residential developments. Yet behind many of these façades, the building work has stopped.