It's very hard and time-consuming to do a reliable stability test. For example file system corruption - that sneaky bastard - might not show up until after days of normal use. There should be many different tests to be performed and the they definitely should include some heavy read/write operations to different memories (rootfs flash/eMMC/microSD) with checksumming.

It does not always work that well in the PC world either, for example ATI overclock auto-tune can find values which are stable in the test but still make some games crash.