In times of the ancients, sans-serif condensed, compressed, ultra-compressed fonts usually had flat sides. If you look at Univers as it gets narrower, curves get squeezed until they become completely flat. These ultra-compressed fonts often had higher stem/gap uniformity. There were strange situations like Helvetica Compressed where the flat sided font barely resembled Helvetica. When I see new compressed fonts, I don't see them going completely flat much anymore. Helvetica Neue retains curved sides but appears to have some degree of gap normalization applied to the heavier compressed weights. My question is: why? Obviously when working with interpolation, a sharp transition from curved to flat requires extra work. Do designers who use type prefer consistent round sides to going totally flat? Nobody cares?
To my eye, there's a certain threshold where flat sides and uniform gaps simply look better. I can't picture Univers Ultra-Compressed with curved sides, no matter how subtle.