Hi
Some of the whizzier aspects of the FCP/SP round-trip process become something of
a liability when dealing with a long project on a tight deadline.
Because SP
gets the FCP timeline clips passed to it there are occasions where if a short edited clip
comes from a much longer source clip-file, then SP wants to render the whole of the
(mostly unused) source clip if you add effects to it. This becomes unwieldy in a lengthy
project.
It can be a lot simpler to work on exact length audio exports from the
Final Cut timeline, which are used to replace the original edited clips in a duplicate
picture-locked Sequence made specially for the sound editor.
As a video editor
who also does the final sound mix I always do the final mix in FCP, using replacement
clips sweetened/sub-mixed externally.
But I'm cautious when there is a deadline
involved, and my Final Cut installation is rock-solid stable - which SP definitely wasn't
in its earlier manifestations

It is becoming increasingly common for different craft specialists to all work on the
same workstation to pull together the project - the video editor, the colourist and
sometimes the sound designer/mixer.
Music is usually created elsewhere, but the
final mix may well be done in the editing workstation.
That gets rid of the
OMF-export problems which frequently arose when working in the old workflow of doing all
the sound design/mix in an external ProTools suite.
In my experience the final
video edit will always benefit from being tweaked to tighten things up once the
constraints of the sound design and music track laying have been idenfified, and that is
the big benefit of FCP<>SP integration