Doesn't spectre report cgg, cgs, cgd as part of the operating point information?

As to the "reliability" perhaps you are asking whether the BSIM3/4 operating point information reported in simulation will match the measurement?  In which case, the answer is often no, but the simulator isn't at fault.  BSIM3/4 models are known to produce non-physical capacitances, including negative transcapacitances.