However, that is not what fpm is. Instead, fpm is a tool to convert
packages from one format to another. That, itself, is not necessarily a
bad idea either; case in point, we do have a package in Debian, "alien",
which provides similar functionality.
What makes fpm a bad idea is that it seems to be based on a stance of
"Debian Policy is just too hard, and I can't be bothered trying to
implement things so they will follow that policy".
If the author says he believes maintainer scripts are "almost always
poorly written shell scripts that break easily"[1], and combining that
with the fact that parts of Debian's infrastructure require stuff being
called from maintainer scripts in order to do something properly, then
that does not inspire much confidence in the resulting packages.