I found an article which expresses some of my discomfort
with Drexler's ideas:

``Flaws in Drexler's vision

It is nevertheless worth examining the shortcomings of
Drexler's original vision because this may give clues as
to how we might make radical nanotechnology feasible. Why,
for example, do illustrations of nanosubmarines look so
absurd to a scientific eye? The reason is that these
pictures assume that the engineering that we employ on
macroscopic scales can simply be scaled down to the
nano-scale. But physics looks very different at such
dimensions. Designs that function well in our macroscopic
world will work less and less well as they shrink in size. [...]''

- http://physicsworld.com/cws/article/print/19961

The article goes on in some detail about how scaling
things down works poorly.

It doesn't contain any real criticism of assemblers, or
diamondoid materials, though.