NiMH will stand being discharged to a much lower voltage than LiPo, so don't need the low voltage protection that LiPo do. As long as the NiMH can supply the current demanded, no problems. On an ESc with LiPo protecion being run on NiMH, a sudden heavy load might well drop the voltage enough to trip the protection.
The bit of circuit in anything that plugs into a radio should be looking for the same type of signal, and before everything was a little computer, such was the case. It mostly still is, but designers are creeping what they think of as cleverness into their systems. Any servo gets its power via (usually) its red and black or brown wires, and its control signal via the white or yellow wire and the black or brown wire. If a servo wants a lot more power than the black or brown wire can deliver, odd things happen to the signal - digital servos generally want more power than their analog equivalents. In boats, there really is no need for them.
When designers are given a permitted range of parameters to play with, they have the unfortunate tendency, rather than aiming for the center of the range of parameters, to go for one end of the limits or the other, usually to keep costs down. Thats OK until an item aiming at one end of the limits runs into one aimed at the other end.
Brushless motors have the reputation that they do demand much higher instantaneous current than brushed motors, but can be more economical with power overall. This means that the system, which includes battery, wiring and ESC, has to be able to deliver the peak current demanded without dropping the supply voltage enough to mess with the timing circuits, which are still as analog as it gets even if the label says "digital".