[Nonfractal]

Being a brass and wire guy I would never choose to elongate the motor mounting holes. I have yet to see a plastic chassis that doesn’t flex/bend in the wrong places hence my questions.
I have almost zero experience with printed chassis however.

I would agree that ideally, elongating motor mount holes is  not a good approach.
Definitely not if you need  vertical and mesh adustments.
A custom mount designed for the job would be better, however sometimes the ideal is just  not available.
I always secure a motor in place with epoxy. 
A little when everything lines up from the factory.
More when the mount has been chopped about in any way.
AlanW