Triggered by the question why should I not use the koma script classes results practically necessarily a consequence question: What packages are incompatible with
scrhack (see documentation in KOMA-Script manual, chapter 16) to fix certain well-known problems with the packages:
scrhackis only for very, very old versions of
hyperrefand does nothing for the current
setspace(in older versions corrections took place, in the current version no corrections are needed),
nomencl: The current version (year 2017) of
scrhackprovides a hack for
nomenclthat not only increases
nomencl's compatibility with KOMA-Script classes but also provides several new features (all from package
nomencl, e.g., automatic running heads or numbered heading. AFAIK this can be used not only in combination with KOMA-Script classes but also in combination with standard classes.
In the question incompatibilities between babelfrench and koma script classes? special problems of
\usepackage[french][babel] are reported.
Which packages do you know having problems with