Patents on things that can exist but don't work well are valid. If they're unsafe, inaccurate, unreliable, etc, that's a problem for commercial viability, but not an issue for getting a patent.
Yeah I was just looking into this and apparently you can even patent things that don't work at all, but may be asked for a working model at some point in the future. Thats surprising, because I could easily see the sense in patenting slightly futuristic ideas with a plausible mechanism and then scrambling to create a model if and when it becomes economically enticing.
A patent could be invalidated if it's something that can't be made, so you can't patent futuristic ideas. Or at least, if you try, and someone calls you on it, it's invalid.
But you can patent inventions that do exist but are crap. Many inventions are crap, and that's okay. If you have a bad invention, you still have an invention.