“Flex” depends on what is tolerable. At Boston’s Museum of Science is a piece of railroad track, to which is attached a strain gauge with a big meter. Children amuse themselves by “bending” the track, perhaps by a few millionths.
When we are polishing to a perfect meet, we are working in the order of microns. Those XS3 machines I used to build had a mast made of 3/4″ 400 series stainless steel casehardened to Rockwell C 60 or so. I can flex that mast if I try.
Masts with bushed sliding tubes cannot have zero-clearance or the tube would not move. This clearance is probably interpreted as mast flex. A solution would be a very expensive two-axis ball bushing.
It depends on the design as well. Masts that are built up from smaller sections illustrate the difference between a “Structure” and a “Mechanism”.
Ideally, we are not using those forces in faceting, but some people are heavy-handed, or are in a hurry.