What I think is: suppose I have a coffee shop and I decide to accept bitcoin for coffee payments (let's leave aside LN), but I will accept the payment as good as soon as I see that the transaction is retransmitted to the blockchan. How likely is it that someone will slip me a transaction that won't be confirmed (and that doesn't have a very low fee, which I could check and reject the transaction based on that).
From what I know, usually payment processors accept 0-confirmation transactions as done only if they don't have RBF.
The problem I see is that a malicious customer may already send you
unconfirmed input. And even if the fee in your direction is just fine, the
unconfirmed parent has a very low fee and has a good chance to get double spent. And when the parent is double spent, the children will become invalid. Of course, the malicious customer has to find the right balance so he doesn't do CPFP by mistake, but that can be done. I think that I've seen that when the network was congested.