It's been my experience that SD cards are almost always what causes a failure on a SBC. Given the cost of the screens, i'd probably choose something that could boot off nvme storage. Or at least tape a new, configured SD card to the case of the SBC for when this inevitably happens.

An SD card is MUCH less reliable than a good hdd unless it's read only.

As someone who works on embedded devices: HDDs are used for media storage and can be easily replaced. Any NAND as a limited life span and good embedded software will try very hard to minimise writes. Though in my particular area, there's additional security constraints on the OS, which preclude any removable flash storage from being used.

They probably expect the signage to change a lot and don't want a hardware failure when they do it too much, or didn't use an external drive in this case and the SD card failed because they wrote to it too much (which would happen eventually anyway).

Even better: Three SD cards with a ZFS mirror and failure notifications

Using an actual hard drive for an embedded system like this would be a failure in and of itself.

You may be surprised to learn that these stores use machines that are occasionally more than a year old and also use inexpensive tech like enterprise spinny disk.

A spinny disk will work in this space, and you know they'll be deciding based on cost.