Power Amplifiers operating in Saturation and VSWR issues
Are you talking about power amplifiers with MOSFETs or BJTs?



  I am talking about power amplifiers with BJTs


BJT saturation region (not the same for MOSFETs): -



Picture from here.

In the saturation region, a BJT is usually regarded as being a low impedance because the current is high and the collector-emitter voltage is low.

What do you mean by immune and what are VSWR effects?



  I meant immune to reflections which can affect the stability of an
  amplifier and even damage it


So, given that the transistor is saturated and presenting a low impedance (much lower than characteristic impedance of the feeder line for instance), any load reflections from the "far end" will be largely re-reflected by the BJT back to the far end.

If the BJT is operating in the "active" (or linear) region, then it is much more susceptible to damage from reflected power because the collector-emitter voltage can grow fairly easily and therefore the reflected power may be largely absorbed by the BJT and turned into heat.