Typing from phone so being brief for now. One time you could write NAND flash pages and have no read errors for months on end. No longer the case now. You need massive amounts of hardware error correction. You have to worry about adjacent cell interactions. You have to use signal processing to read degraded bits. Then there are weird eratta that will not be disclosed to you unless you have an NDA and beg to them. This is just a few things that come to mind but there are tons more.