Drum level and combustion control

between ratio control, cascade control, and feedforward control which of these is best for Drum Level and combustion control
Of the systems that I have worked with, drum level is cascade control with feedforward trim and combustion control is ratio+ cascade + feedforward trim but then again it all depends upon the consultant and to the OEM about how they want to implement it. But generally it is a mix of different control methods with override logics as no single stand alone control method is feasible.