I found The Machinery of Freedom to be rather speculative, particularly wrt. privatized law. Friedman also seems to handwave over natural-monopoly and public-good problems, although he sensibly describes them as "hard".

Still, David Friedman is far better than such "Austrian economists" as Murray Rothbard, who by his own admission was not engaging in intellectually-honest scholarship at all, but simply seeking convincing arguments for his anarcho-capitalist, propertarian ideology.