On top of that, hard crashes and partial brownouts due to undervolting can and do lead to data corruption on all sorts of storage, no matter if eMMC, straight NAND or microSD cards.

Most phone PCB designs do not account properly for the scenario "battery gone bad leads to undervolting of components" at all, the best you're gonna get is the BMC cutting off everything when the voltage at the battery drops way too much - but that is a failsafe mechanism, the flash, memory and processor chips will have undergone brownout events before the BMC emergency shutdown hits.