The only issue I got with contrast changes is that even at highest contrast we still see stuff on-calc, while on emulators we still see stuff sometimes at lowest contrast. This is why it was always so hard to take animated screenshots of my 2003-05 BASIC RPGs. Plus if you switch contrast rapidly between light and dark it creates bluescale for a split second on 15 MHz models, which can either be a desired effect if used for magic animations, for example, or undesired due to the potential of wearing down your LCD if abused.
Also some calcs have different contrast levels, which is why for Reuben series I decided to let the user choose his default contrast.