The problem with lisp and other modern languages are
the issues with garbage
collection which might arise in real time use.
I don't think this is a problem; you can usually turn off
the GC during time-critical code, and in most cases, such code
is in C anyway (handled via a foreign function interface), and
at least in normal use, you're in control at that level.
Besides, modern hardware is far faster than people seem to
realize, and modern GC's don't spend much time on a sweep.
Gad, now I'll have to go and run some tests...