"still confused and wondering what the thinking behind the mounting position limitations was based on. "
Most people are - there are a few reasons why manufacturers do what they do, and unfortunately some of them are driven by cost of manufacturing -
from a user's standpoint, the "bad" things are circuit boards that are too large to be well supported against vibration and bumps - repeated flexing will shorten the life of the printed traces on the board, so early failure.
Ways around that are staying with smaller individual boards, rather than trying to cram ALL the functions of that piece of gear onto one LARGE board - if a company recommends their unit gets mounted only in one plane, there's a good chance they did EXACTLY THAT and are trying to compensate for it by mounting that too-large board so it's standing on edge - this lessens the effect of gravity/bumps, etc, on FLEX.
If circuit boards are kept narrower, in at least one axis, then they're less likely to flex in transit and can sit either horizontally OR vertically, with little difference in longevity. Still, for mobile equipment it's better if the internal boards end up sitting vertically if possible. Also, vertical boards are better able to take advantage of any convective cooling, so a bit less likely to "let the magic smoke out" if a cooling fan fails.
If one manufacturer's similar unit costs more than another, it's probably either greed or they've actually hired people who KNOW what's the better way to do things, and that way costs more.
A couple of my "past lives" jobs entailed design/build/test of proprietary parameter testing equipment, and more/smaller was one of our main objectives when designing boards for that gear.
However, nearly ALL my experience in the design area pre-dates SMT (SurfaceMountTechnology), so SOME of the above may not be quite as important... Steve