One reason why people often assume flexible nibs produce
smoother writing is because flex nibs are often referred to
as "soft" nibs. However, the "soft" really
just refers to the metal and the fact that the tines bend
(flex) under pressure.
In reality, however, flexible nibs are trickier to write
with. As a nib is "flexed" the tines are spread,
exposing the sharp inner margins of the tipping to the paper.
With the tines splayed, and working independently of eachother
as they are pushed across the paper, there is more chance
for the tipping to catch the fibers of the paper. Flexible
nibs are more prone to scratchiness than smoothness.
Though flexible nibs are great for calligraphy work, and
producing line width variation, they are not necessarily considered
smooth writers.