Already been there and done that.
The coil meter is not very helpful because it does not store the worst imbalance. You could throw away everything and simply plug a multimeter on the two drains, or a scope, same principle as the coil meter, I checked visually the imbalance on two traces on the scope screen, as the bias changed, and I can see the distance between the two traces, the imbalance. But then, I need pen and paper to write down the size of the imbalance as I go along trying to find a good match, and if I get interrupted I will not remember what the imbalance actually was (unless I have been keeping notes). It's like the peak detectors that come with VU meters on mixing decks and consoles - without them you'd need pen and paper to write down the levels across different passages.
This little device does this for you, the position of the Tolerance pot is a marker for the "best match so far" and you do not need to write down anything, no need for scope or coil meter or multimeter, no need to remember anything and is very accurate as well. And in terms of complexity, well, two op-amps, 21 resistors and 5 capacitors, that is not complex is it? I am planning to put in inside a small case, 8x9 cm, to resemble a small, handheld tool.