"As long as operators delete keys"
Honestly, I'm not sure how anyone makes it past this sentence. If I'm going to trust them to delete keys, why not just go custodial?
A tree of "unilateral exits"
They have a lot of complicated key rewriting making it difficult to understand, but any scheme of this nature needs to prove it isn't vulnerable to an early transaction forcing a whole subtree on-chain to defend funds. Your 15,000â‚¿ wallet isn't worth anything if someone waits for a high fee environment and it takes 10,000â‚¿ to defend, plus another UTXO to get back in.
But I've been hacking on something that I think is better, so there's some conflict of interest here