They actually do work just fine, but are a pain in the tucus to use. A dedicated mortiser has a single lever that a single pull travels the bit and chisel the whole way. It is really awkward turning and turning and turning that crank on a drill press to get the bit/chisel to depth. The add-ons also don't have as good (or accurate) a hold-down as a dedicated mortiser.
Bottom line - they can be made to work - it is just harder than a dedicated machine.
Powermatic is the top of the line benchtop mortiser if you can afford one. I found mine on CL and sold the Delta add-on also on CL.