Can MCPCBs be used for audio frequency and/or MCU clock frequency applications?
Generally, you're right: That metal plane is typically nothing but a big ground plane. 

There's one electrical difference: That thick ground plane is typically only the thickness of some isolating paint away from the trace, which is far less than the 0.8mm – 1.6 mm of the average fiberglass substrate thickness of two-layer PCBs.

That makes a difference for high-frequency circuitry, where the wave impedance of a trace matters, and the signal isn't actually carried by the conducting trace, but as wave in the isolator.

However, that's RF, not 20 MHz digital – you should be fine.

Note that MCPCBs are typically a pain to solder by hand (exactly the reason you use them – they carry heat away fast), and that you can't have vias through them – and that really limits their usefulness!