It would not run the motor to its full potential, also not all FETS fail equally per phase meaning A phase + may have 4 working FETS B phase - may only have 1 working FET, the reaming FETS may all sill be OK thus a surge in power output.

It would also blow the reaming FETS stating with the weakest phase.

Also note when FETS fail, they do not always fail in the open circuit state, that would allow you to continue using the ESC. They can fail in a short from Drain to Sink or either to Gate. this can cause excessive heat to the ESC, poor rpm range of the motor, motor not revving smoothly ect.