I'd say that doing due diligence to test the card is wise, but once the card has a track record of storing data, there's no reason to believe flash memory isn't at least as reliable as a HDD for long-term storage. Other than their connector pins, flash cards have no moving parts to wear out, no bearings to seize up, no read/write heads to crash into a spinning platter, etc. A flash card is far more likely than a HDD to survive submersion, prolonged exposure to high humidity, extreme temperatures, or high G forces. The flash cells are subject to a finite number of write cycles before failure, but they are sealed inside the chip packaging and there's no limit to the number of times data may be read from the cells. Flash cards also generate far less heat than a HDD.
Most unrecoverable HDD data loss is caused either by a mechanical problem (head crash, bearing failure, condensation on drive platter, etc.) or component failure due to excessive heat buildup--things that don't really apply to flash memory devices. The only thing that might keep flash from trouncing HDD reliability-wise as an archival medium is some as-yet-undiscovered factor causing chip failure after several decades of storage. But I haven't seen any evidence so far to indicate such a factor exists, so overall I'd say it's safe to assume you're no more likely to lose data archived on flash cards than on hard drives.