[jsnell]

Yes, the range of applications where proof of work is viable is really narrow. (In fact, so narrow that I suspect it can't work for anything where the abuse has a monetary motive.)

One way to think about this is by comparing the cost of passing the POW to the money the same compute resources would make when mining a cryptocurrency. I believe that a low-end phone used for mining a CPU-based cryptocurrency would be making O(1 cent) per day. Let's say that you're willing to cause 1 minute of friction for legit users on low-end devices (already something that I'd expect will be unacceptable from a product perspective). Congratulations: you just cost the attacker 1/1500th of a cent. That's orders of magnitudes too low to have any impact on the economics of spam, credential stuffing, scraping, or other typical bulk abuse.