I think however well they design the interior if the train is built for shorter distances it doesn't make it really suitable to run a long distance service. For example, the ATW 150s have interiors similar to the refurbished 156s but that doesn't make them suitable for Manchester to South Wales or South Wales to North Wales services.
You are right, I would not want to endure a South-North Wales journey on a 150! But at least they have toilets!