Since the re-introduction of scale-wheeled locomotives (-2 suffix) MTH has tried to move closer to prototype accuracy. This is in part due to the 3RS movement. I would look to recent releases over the past six catalogs (three years) to see if there's a trend. Usually, in each successive catalog there are small improvements. Keep in mind that diesel offerings are based on the 3-rail versions, so there are going to be issues like truck side frames sitting too far out (which can be corrected with precise drilling and some filing so the brake shoes don't touch the wheels). Some details, such as windshield wipers are plastic and a bit thick, but these could be replaced. It all comes down to what you're willing to modify on an out-of-the-box model to bring it closer to prototype. I have a couple of GP30's that I'm working on right now.
Also note that Proto-3, the command/sound system in current MTH production supports DCC operation. Low speed operation is good at 3 scale miles per hour out of the box and improves a bit after break-in.
For contrast compare the U25B (2009) with the GP38-2 (2015) below:
The U25B was a bit generic in its configuration, but they still got pretty close to prototype. In this case, I re-numbered the locomotive from 6608 to 6609.
Compare to the prototype:
The newly-released GP38-2 (ATSF had 11 from TP&W) has road-specific details, including the roof-top A/C unit and proper placement of the warning beacon and horn array. I can't find the photo I had of #2376 and #2380 which did have the horns off to the engineer side of the cab. 2373 shows where they installed the warning beacon.