Certainly the technology is viable, but perhaps more viable at a community scale, rather than at an individual scale.I wonder where those numbers come from. The data I've looked at from multiple sources uses a lot less manure (if that's the feedstock to be used) for gas production. Besides, manure isn't the only thing that can be made into biogas. Just about anything that biodegrades can. Grass, for example. Kitchen scraps. Wood. Paper. Just about any vegetable matter. Fat (animal or vegetable derived) is much more energy dense and also as a consequence produces more biogas.
This is tech that used to be used by the West until the Rockefeller monopoly basically took over, but it has been (and still is) used in places like India and China for over 200 years, either on a community scale or just for a small family. It's also used in Africa. If it were so nonviable for the average person without 77 pigs or 11 cows, it wouldn't be used, now would it?