There are just too many of them.
Some products are faulty by design (Davis weather stations).
Yet others are just in need of a little help (VSDControl for class A AIS units).
There are even nice products which are just a bit lacking in the documentation department (Airmar H2183 / Si-Tex HDK-11 heading sensor).
Products from certain equipment vendors seem to require expensive add-ons for very simple tasks (Standard Horizon HX300 marine VHF radio).