For a lower voltage drop one could use Schottky diodes, but what are the disadvantages of Schottky's?

They cost more, have higher reverse leakage current, and are physically larger according to a quick search. Of course they're much faster though :)

Looks like in a same size comparison they can't dissipate as much power as a typical power diode. Also with larger currents you lose that Vfw advantage. Oh and wiki says they normally have lower reverse voltage rating on the order of 50V.

Far from a comprehensive list:

• Schottky diodes of a comparable rating are generally more expensive than PN silicon diodes. I've seen price differences of 20% - 200% depending on the rating.
• Schottky diodes have a lower maximum reverse voltage rating than is possible with PN diodes.

For essentially the same reason that schottkys have low forward drop, they have large reverse currents.

From the diode equation:

\$I_f = I_s\cdot e^{\frac{-qV_f}{kT}}, V_f = \frac{kT} {q}\cdot \ln\frac{I_f}{I_s}\$

-- having a large Is term is what makes Vf small. However, the reverse leakage current is also equal to the Is value.

From their structure, silicon schottkys can only withstand about -30 V alone. Higher voltage ones are created, but basically these have an internal JFET in series with them -- this is what actually withstands most of the reverse voltage.