Recently I was asked to build an HO scale SD70M for a UP engineer. I will say this scale is very different to work in than my usual N Scale. The project was a challenge, but I think it turned out well.
This was the engine that the engineer had certified in, I started with an Athearn unit, redid the roof details, added cab shades and mirrors, renumbered the engine and weathered it to look like the prototype.
It was fun, but I won't be switching scales any time soon.