I have saw a few shitty custom models out there. Specially on 1:400.
I have seen some crappy ones indeed. Basically it's like I said before: it depends on the quality of decals (and the printer used!), the quality of the paint, the skills of the custom-maker...
Here are two custom models I own, for example:
Itapemirim Cargo B727-200F - donor model was a GJ Planet Airways
Pic of the real plane here, if you wish to compare.

VASP A300 - donor was a Dragon Wings Carnival Airlines
Pic of the real plane here, if you wish to compare

These are quick phone pics, so lighting and photo quality aren't ideal, but IMHO these customs look excellent. No crooked decal placement, no smudges, correct colors and a great finish. The may not look 100% neat like factory models but can come quite close.
here are both models "out in the wild" in my diorama. They blend in perfectly with other models.

