Thanks @Pascal-Ramsey! Yes, I after discovering research on UPHS showing that it could be highly feasible, I spent much of the summer working on that feasibility study to summarize and contextualize my findings.
Terrament's biggest obstacle at the moment is getting funding. But we're working with civil engineers to flesh out our designs and strengthen our case so we can pitch funders in earnest. We're really just getting started.
As far as major obstacles for UPHS in general... The only big obstacle is finding the right partner willing to make a massive, long term, profitable investment. This partner could be a municipality, a Transmission System Operator, a cooperative pool of power generators, or some other entity.
UPHS doesn't use any new technology. Everything from the tunnel boring to the hydro mechanics is very well understood stuff. So govs and power industry folks just need to think big, understand this math, and start building this as fast as possible. We need a lot of Li-ion batteries also, but swapping in UPHS for some of largest bulk storage could save us trillions of dollars on our impending energy storage crisis.