on a scale of 0-9, I'd put factory reset at about a 5. I'd wager there's still software out there that could do a raw read of the SD storage device's blocks. Unless Apple and friends have the device reset go to extra measures that tell the drive (at the chip-level, not the OS) "report all zeros" when read from, I think to increase your safety to "level 9" you should overwrite the disk by copying a large file several times until all the free space is consumed. Even then, flash drives have more physical storage than they report, but accessing those blocks would be pressing the edge of whats practical except in the most exigent situations.

Edit: I didn't realize virtually all phones' storage is encrypted nowadays.