Perma-shadow only works if there is no healing and absolutely no signets, skills, or PBAoE skills that remove enchants. Just keep that in mind when you choose your area.
Correct. You'll want to investigate the area you're going to before (check out a beastiary on wiki) you try to run a perma-Shadow. It can do stuff like Vermin, UW and parts of FoW pretty well, as well as making for a killer chest-runner through HM FoW and some other high-end areas. I'm trying to find some spots for the Moebius Blossom critical defenses build... it's fun to play and feels more like a real farmer, but it's pretty limited in scope. Still, if you've got both those builds you can find some places to do, though they might get boring after a while ^_^.