IDEfix is a real replacement for the internal IDE driver (scsi.device) which gives real support for 64bit commands. NSDPatch only gives 64bit-command to SCSI-command translation, using the original driver with all its bugs. This allows you to use only HDDs up to 8GB, not more (due to driver bugs).
But that's not the reason why people don't like NSD. A full implementation of NSD would also give real 64bit support (in fact IDEfix does implement both TD64 and NSD).
There is a collection of articles about why NSD is bad on the bottom of Ralph Babel's home page:
http://babel.org/amiga/Bye,
Thomas