I'm not sure a system like this is going to be good for mosses. The main problem being the evenness of the light. For documenting coins and jewelery the ring light is probably alright, but I can't imagine being able to get interesting light from it.
You'll likely get better results out of a cheap home made beer can reflector.
I don’t see much of a downside. LED lights are pretty bright these days, so you might not even have to crank your ISO up all that much for hand-held stuff. My only recommendation would be to do a colour test before you get it out into the real world. Go to a photo store (someplace that deals in pro gear, not just a photofinishing place) and buy a standard grey card. Shoot that with the ring light and use it to set a custom white balance, just in case the LEDs have a bit of colour cast to them.