In the short term I'm sure the approach works, but the proof of work part of it is just theatrics and doesn't stand up to scrutiny. Using an uncommon CAPTCHA with some alternative heuristics is enough to deter spammers (until it isn't).
Any back of the napkin calculation will show you that if spammers actually start solving the challenge, they will either pay a ridiculously low amount of server CPU time, or regular users on relatively underpowered devices (i.e. smartphones) will have to wait forever for a pageload.
You'll never see that basic calculation being spelled out on product pages, because it will show its mostly smoke and mirrors.
Any back of the napkin calculation will show you that if spammers actually start solving the challenge, they will either pay a ridiculously low amount of server CPU time, or regular users on relatively underpowered devices (i.e. smartphones) will have to wait forever for a pageload.
You'll never see that basic calculation being spelled out on product pages, because it will show its mostly smoke and mirrors.