"Lower voltage swing from the same voltage supply compared to the BJT implies lower gain" - No it has nothing to do with gain but with characteristic.
"Lower gain (for the same amount of bias current) compared to the BJT implies lower bandwidth". - No it doesn't. Even between bipolar transistors current gain has nothing to do with max bandwidth. It is just matter of design. Mosfets are generally much faster.
"I also don't know if there is any substantial evidence of BJTs being less robust than MOSFETs" - sure they are more robust since Mosfets don't suffer secondary breakdown (hot spots of current on the wafer with higher voltages resulting in thermal avalanche - very nasty complication) and are thermally stable (BJT's Vbe drops with temperature increasing collector current hence increasing temperature - thermal runaway). Mosfet drain-source resistance has a positive temperature coefficient, so they tend to be self protective.
As for tubey sound - Mosfets have softer clipping than BJts but are more nonlinear and require much higher gain before feedback to correct it - in result behave pretty much the same as BJts. Higher gain results in design more prone to TIM and therefore higher order odd harmonics.
Just think about it - if one type of devices would have clear advantage nobody would use other devices. Some manufacturers (Krell, Levinson etc) use bipolar designs.
"Lower gain (for the same amount of bias current) compared to the BJT implies lower bandwidth". - No it doesn't. Even between bipolar transistors current gain has nothing to do with max bandwidth. It is just matter of design. Mosfets are generally much faster.
"I also don't know if there is any substantial evidence of BJTs being less robust than MOSFETs" - sure they are more robust since Mosfets don't suffer secondary breakdown (hot spots of current on the wafer with higher voltages resulting in thermal avalanche - very nasty complication) and are thermally stable (BJT's Vbe drops with temperature increasing collector current hence increasing temperature - thermal runaway). Mosfet drain-source resistance has a positive temperature coefficient, so they tend to be self protective.
As for tubey sound - Mosfets have softer clipping than BJts but are more nonlinear and require much higher gain before feedback to correct it - in result behave pretty much the same as BJts. Higher gain results in design more prone to TIM and therefore higher order odd harmonics.
Just think about it - if one type of devices would have clear advantage nobody would use other devices. Some manufacturers (Krell, Levinson etc) use bipolar designs.