Do you think they are trying to do too much with a single engine? Seems to me they should have separate products for 3 rail and 2 rail. I think at one time MTH made a limited number of 2 rail engines.
Also, one of the suggestions in the Tips and Tricks publication from Kalmbach a few years ago had to do with insulating one of the outside rails to allow for future accessory activation. I did that on my layout to the outside third rail to allow for the installation of crossing gates ect.
My guess is that could cause a problem if only one of the outside rails of one of these MTH engines can pick up the common or ground.