What are the advantages of GaN on SiC?

Let's say simply that SiC is a very dissipative substrate compared to Si. Devices on this substrate can operate at a high voltage and high drain current. Junction temperature will increase slowly with RF power and consequently RF performance (Gain, PAE) will ultimately be better. Reliability and life time of a device at the same dissipation conditions are better for SiC.

Another point to be mentioned is that SiC and GaN are lattice-matched, so no cristalographic defect is created at the SiC-GaN interface.

The last point is linked to the high resistivity of SiC: this is very beneficial for mm-wave propagation, which is needed when you design high frequency MMICs with large matching circuits.