I expect a lot of phones are using eMMC, due to it's lower pin counts compared to raw flash. eMMC presents a SCSI/block interface to the OS and so has it's own block translation and wear-leveling firmware which hides the underlying flash.
The problem with this is exactly that however - you're depending on the quality of the eMMC firmware, which can be dodgy. Look at the problems the SSD drives have had of falling off a performance cliff. Also, we've had issues with eMMC reliability from some vendors. I don't know if you can update that firmware on the fly or not.
We're to the point of putting bare NAND back on our boards and using UBIFS instead.