Do you mean my last point? What I am saying is that a flexible TPU filament such as NinjaFlex will not neatly feed into the extruder, then from the first hobbed gear to the second one, and then out to the bowden tube, if you do not put a channel or some tubing in to control where it goes.
I think what you said makes sense,TPU filament should be a problem.I prepare for a test and fix my extruder:P