I definitely prefer mast-arm signals to be mounted on brackets, I think signals hanging on a chain looks a bit lazy, and they blow around too much.
That said, agencies do sometimes make a mess of bracket-mounted mast-arm signals. That Illinois signal above and, to a lesser extent, the NJ signal posted by
roadman65, aren't particularly pleasing to look at. The exposed wiring in the NJ example, and the leftover mounting equipment in the Illinois example, aren't great, though I prefer the location of the signal
now at that Illinois example; before it looks a bit awkward hanging that low.
In whatever way it may be applicable, I think more agencies should take a page out of Japan's design manual; the horizontal signals may be odious to some, but I'll be damned if the installations aren't super tidy:
https://goo.gl/maps/VQ3rEmrWx4tBi4637. I do wish more post-mounted signals were used, though.