RE: Servo Reversers.... how do they work?
Given a sound design, there are no downsides to a servo reverser other than it is another component that may fail.
I have used them now and then and never had a problem.
In principle the design is very simple. A servo gets its position information from the length of a pulse that is repeated many times every second. The pulse length varies between 1.0 and 2.0 ms with 1.5 ms representing the neutral. In order to reverse the servo we need a circuit that converts a 1 ms signal into a 2 ms signal and vice versa.
In order to accomplish this a typical servo reverser produces a 3 ms pulse and subtracts the incoming pulse from that value.
A 1 ms input pulse produces a 3-1=2 ms output pulse and a 2 ms input pulse produces a 3-2=1 ms pulse, exactly what is needed to reverse the direction of the servo.
For the technically inclined: The easiest way to accomplish the subtraction is to do a logical XOR between the input signal and a 3 ms reference pulse that is triggerd by the rising edge of the input signal.