Never good when these things happen
However is there a way to reduce the NVH on these more simple gearboxes i wonder?
Maybe a rubber isolator pad under the gearbox sandwhiched between the metal casing and the tank hull would help slightly?
I just wonder if a cover could maybe be fashioned also over the top with a isolator material to reduce audible noise.
The noise from these units though does tend to be due to poor meshing of gears with not so great profile and they run loud.
As for the other gearboxes which slips or doesn't move till it has alot of throttle. One sounds like a spline maybe and the gear has slipped out of mesh or the spline has shredded and the gear spins sort of freely on the shaft. I don't know how well these gearboxes split apart, but if it already broke i would just go for it and try methodically to lay it out in order of removing the part and make sure you lay it out in the correct orientation so you can re-assemble.
As for the other one which jumps, that sounds like something is out of round or bent. Causing alot of restriction until the higher current allows a greater level of torque to overcome the restriction. Rather than being bent however, it maybe a simple thing like a bearing is worn so the gear mesh is out of alignment or a bush is not seated correctly.
Although i have also seen on cheap gears that they are machine so they have alot of runout and they do not run true. In which case you are forced to do drastic machining or replacement.
Unfortunately these things are made for mass production volume rather than good quality

Sorry if that hasn't helped