Even at herbalism 300, my wife sometimes gets the "failed" message when she tries to pick certain herbs. Usually, when she repeats the operation she gets them. The +5 herbalism will prevent that. Just makes things a little quicker. It doesn't mean you get more or better herbs, just that you don't fail when you're trying to pick them as often.
I think I know what you mean by the herbs disappearing - the little yellow spot on your minimap disappears, then when you go towards where you saw it, it reappears? I notice the same thing with mining - I don't know if it has to do with intervening terrain or if it is something to do with the way the program is written.