One big disadvantage of solar thermal is that it doesn't work when it's cloudy. Conventional (photovoltaic) solar panels work fine with the diffuse light from an overcast sky, but you cannot focus diffuse light to create high temperature.

It's also mechanically complicated. Not only do you need a turbine (or Stirling engine) but you also need a tracking device to adjust the mirror angle continuously. Whereas conventional solar panels have no moving parts at all.