Why should I use otfinstall instead of texfont, besides the fact that texfont doesn't work?
Do you really need another reason? Have you tried otfinst? The point of all these scripts is to make your life easier in generating metrics, auxiliary fonts and map files; if you tear yourself with questions before trying them much of their interest is lost on you.
Is texfont incomplete?
I have no idea; but at any rate it belongs to a clearly “older” generation of scripts. The latest available version has “version=2004.02.06” in the documentation part, and even without that you can spot it quite easily since it belongs to Hans' Perl period ;-)
Or am I doing something wrong?
Probably not.
What are the advantages/downsides of both?
Why don't you try it for yourself? Arthur