Probably implementation hell. The big idea theoretically works, but when you get into the details the compromises for implementation steal away the gains. I'm

Probably implementation hell. The big idea theoretically works, but when you get into the details the compromises for implementation steal away the gains. I'm always reminded of the rotary internal combustion engine: in theory there's a whole suite of benefits, in practice they're "interesting" but not that great once practically built.