I'm not sure there is one - thoughts: (1) There's lots written about specific "dimensions" of hardness. How reliable is X cryptography? What are Ethereum's defenses against certain attacks? What should long-term issuance be? Much of ethresear.ch is about narrow pieces that contribute to Ethereum's hardness.
(2) That makes sense for an ecosystem that has been focused on learning about and evolving the protocol to improve it. Over time, I believe our ecosystem needs significantly shift towards translating that knowledge to outsiders - see https://stark.mirror.xyz/rkLEVz9p4r3ouusD-WCkWP_iVZYkZ0K7TFkzeRfiXCU