Negatives have a much more resrtricted dynamic range, which means
you have a better chance on a flatbed to capture everything you
need as compared to scanning a transparency.
While color negs have a very low dynamic range, some B&W negs (like
TMAX100) have a very high dynamic rage sometimes exceeding 5.0.
The big advantage of a drum scanner is the increased sensitivity to
light, which helps a lot in scanning transparencies that have deep
shadow detail.
While it's one thing to get excellent detail across a high dynamic
range, it's another thing to get excellent detail across a very
narrow dynamic range. Color C-41 negs have a very narrow range. To
take things to an extreme, let's say we are dealing with a B&W neg
that is 6+ stops under or over exposed. This hypothetical neg would
be nearly clear or black and only a drum scanner would be able to
pull so much detail out of it.
I remember a particular, black "bulletproof" neg from 1995 that
wouldn't print in the wet darkroom with 20+ minute exposure times nor
scan on a CCD scanner but the drum scanner I ran was able to produce
a final file that looked quite normal. It was a shining but rare
example of drum scan superiority.
