The problem is the unpredictability. In my real-world tests involving Aqua wallet users who tried to pay a Lightning invoice in a retail environment, I saw more than one instance of the payment failing to reach the destination, but on the sender’s side it showed as completed.

This causes a huge headache for both the sender and receiver. In an in-person setting such as a restaurant or store, this means a customer awkwardly waiting for up to an hour before the invoice fully times out with no indication of what went wrong during this entire time. As far as the customer is concerned, their payment was completed and their wallet balance is reduced. As far as the merchant is concerned, no payment was received and they then have to decide what to do next.

You can imagine how this will not scale for a commercial setting. A merchant can’t just say, “You can pay with Lightning, but not that particular way, because it’s unreliable.” This is a potential adoption nightmare.