Structural integrity is part of what I described.
So is film flatness really.
It’s all about aligning the lens focal plane with the film plane.
The imperfections all add up. In lucky cases they can somewhat cancel each other out, if alignment is right.
In other (most) cases they will contribute to a much worse result than you’d think reasonable from just looking at the single ailments.
The reason a view camera works is because the photographer meticulously goes over the ground glass, with a loupe, correcting small focus errors and mostly stops down a lot.
On a folder full open, and with a close-ish subject there is no way to check alignment even though the slightest lack of alignment potentially can throw the whole photo out of focus.
The much lusted after unit focusing folders, run into problems with alignment, for much the same reasons.
Unit focus is really not that hard to do mechanically.
It is hard to do right in a folder though.
Agfa and Zeiss Ikon had good reason to insist on front cell focusing (other than cost).
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