The battery is the problem. When manufacturing a PCB, adding a battery such as for a RTC or even a chip/socket RAM with built in battery causes a concern if/when the part is "wave soldered" because the terminals of the battery are momentarily shorted out in the solder wave.

Most PCB batteries state that they can survive a wave solder process with diminished battery power but recommend that you hand-solder the part for best results. That's extra labor and cost but that's what we do with our RTC battery.

Flash requires no battery and has no residual pin voltages when powered down. If for no other reason than PCB manufacturing, Flash is a real advantage.