SmartScan is genuinely reliable for a clean photo of a typical retail or restaurant receipt. It's noticeably less reliable on faded thermal paper, handwritten receipts, or multi-page hotel folios, and it has no way to catch a purchase that simply never got a photo taken before the paper was thrown out.
Email forwarding works well for anyone disciplined enough to use it, but most of the receipts that go missing come from channels nobody thinks to forward: a vendor's invoice that lands in a shared accounts-payable inbox, a subscription renewal charge with no receipt email at all, a purchase made by someone other than the cardholder.
The other real gap: SmartScan reads what's on the receipt, not what it should be coded to. It doesn't know which client or project a lunch should bill against unless a person tags it, which is exactly the kind of coding work that's easy to get wrong or skip under time pressure.