To be clear, Jack Mallers works on Strike, which uses the lightning network. Settlement is final on receipt, which is limited by network latency (and db commit time).
The cost is that it's possible for your direct counterparty to attempt to cheat: you've been trading signed bitcoin transactions all this time, and they could try to use an old one. For this reason there's a lockout period for any unilateral onchain settlement (typically 1 - 14 days), and if they attempt to cheat you have that time to penalize them (they gave you a key to do this when you agreed the balance in the lightning network was updated). This penalty can be outsourced, too.
The cost is that it's possible for your direct counterparty to attempt to cheat: you've been trading signed bitcoin transactions all this time, and they could try to use an old one. For this reason there's a lockout period for any unilateral onchain settlement (typically 1 - 14 days), and if they attempt to cheat you have that time to penalize them (they gave you a key to do this when you agreed the balance in the lightning network was updated). This penalty can be outsourced, too.