"Blind" charging of batteries in series, regardless of chemistry, is a bad idea. By "blind" charging I refer to the treating of multiple series cells as one single battery.
However, blind charging is cheap and simple. If you don't care too much about the longevity of the cells, but you do care about the cost and space, then blind charging of multiple cells is an option.
Most multi-cell chargers don't blind charge. They treat each cell as a separate entity and tailor the charging to ensure proper charging of all the cells in the chain.
You will notice that in the schematics for all the chips of this type that there is more that just two wires connecting the charger to the battery - there is a third that connects to the mid-point between the batteries (or two more when it's a 3-cell charger). That allows the chip to monitor the state of each individual cell in the chain and affect each cell separately.
It's not 100% perfect, but it's a darn sight better than blind charging, and sure beats removing the batteries from a device to manually charge each one separately.
As you blind charge cells in series the cell with the most charge triggers the end of charge, so that cell is always fully charged. The other cell hasn't fully charged, and never fully charges. This is the source of the cell imbalance.
Over time that second cell's voltage drops further and further until it gets to a level where it can no longer be charged back up even if you were to take it out of the circuit and charge it separately. It's now dead.
So while blind charging series cells does cause cells to die over time, it's not a catastrophic (as in the blowing body parts across the room catastrophic) failure, but a gradual diminishing of charge until it's all gone.
You'll notice a key couple of phrases in the data sheet, especially under the Typical Applications section:
- Low-Cost LiFePO 4 Battery Chargers
Cheap products where you care more about how much it costs to make them than how good they are. You don't care that the you car's battery will be useless in 6 months - the kid will be bored with it by boxing day anyway. If they really wanted a good remote control car they'd get a real one, not a toy, but of course that'd cost more.
So the bottom line, I reiterate, yet again, is: