My failure was caused by a pinhole leak in the gas tank causing the bellows to be immersed in gasoline. This dissolved the rubber part of the bellows and left schmutzy, flimsy canvas backing in place. It happened on the hard but the boat would have sunk at the mooring.
Also, the rings or faces of the seal eventually wear and have to be replaced. This requires removing the coupling from the shaft, which is a huge pain.
My installation has never been bone-dry either. And I worry that some grit, dropped tool, or misguided boat hook from the bilge or lazarette will get between the faces of the seal, or misalign the bellows, and cause a leak.
And now - you mention engine heat. Great, I hadn't worried about that before. Add that to the list. Thanks.
With the stuffing box, nothing is fragile and all the bits are pretty much wrapped in that indestructible thick hose anyway. The hose and bronze can last for decades, the hose clamps can be replaced without removing anything else, and the flax packing can be replaced with a minimum of disassembly, time, or cost. And Teflon packing might last as long as the rest of the system.