Reports are circulating that the recent failures of 128 GB iPhone 6 Plus models are due to the use of TLC NAND. Apple is the world's largest flash consumer, and has made a concerted effort to utilize technology to expand their use of less-desirable flash (ie 'cheap'). TLC NAND, which holds more bits per cell than MLC NAND, has a higher capacity than other types of NAND. Using cheaper flash provides Apple with better margins, and also helps during periodic flash shortages by expanding the pool of flash they can purchase.
Apple actually purchased Anobit in 2012 to integrate their MSP (Memory Signal Processing) technology, which extends endurance for less-desirable flash. It is rumored that the 128GB iPhone is one of the few select models that utilizes TLC NAND, and users are reporting widespread crashes or reboot loops. This problem has persisted even after the latest update to iOS 8.1. The problem may be confined to the NAND controller on the iPhone, but with no direct confirmation from Apple, users are left to speculate.
This news comes on the heels of problems with TLC-powered Samsung SSDs, but that problem was also relegated to a firmware bug. TLC is becoming a widely-used form of NAND, and NetApp is even developing TLC platforms for datacenter usage, so these teething problems are likely specific issues with endurance-extending techniques, such as DSP, wear leveling, and read-retry/disturb.
Compounding the issue for Apple is the continuing Bendgate scandal, as more and more users are reporting iPhones bending in their pockets.