This reminds me that we should probably sign on to the Unseelie Accords at some point in the near-to-mid-ish future.
Why? The main benefits of signing the Accords are that Mab won't resent you from not joining her pet project, access to diplomatic contacts and procedures, and stability. We are already cool with Mab (pun intended), we have plenty of contacts built on mutual benefit and interests (and have shown that we have enough to offer that we can easily make new ones), and stability is a two-edged sword for somebody as powerful as us that wants to change the status quo.
Because there
might be a clause for just outright declaring a war of conquest on another member of Accords, but in canon we know that even the Red Court strongly preferred to seize a convenient casus belli first. Which we would need to manufacture.
Red Court is a signatory, so is technically Black Court, Drakul, Denarians as group (except for Nicodemus). So say goodbye to element of surprise, when we would at the very least need to formally declare war before attacking any of them (and we are planning an ambush on Denarians already). And any human thralls or slaves they have? Unless we are formally at war, then it would be violation of Accords to rescue them, because you would be "stealing" from another faction, which is an act of aggression. Remember our attack on Duchess Arianna? It wouldn't have been possible like that, had we been signatories.
Our enemies will manipulate our morals into violating the Accords and that will hurt our relationship with Mab more than not just joining in the first place. We could try to play the duel and wergild games to mitigate violations, but why?
There's a damn good reason why The Knights and The Church aren't members.