Tesla has good QC and yet its multi-thousand buck cars have burnt to a crisp thanks to a faulty cell or two. Phones and laptops burn. Hoverboards - you are literally walking on fire...
There is no reason that a Bosch or other battery pack wouldn't spontaneously burst into flames if a Li-ion cell has been having a bad day. Li-ion cells pack huge amounts of energy into a small space that is why they are in a metal casing. The notion that Li-ion bike batteries are safe as houses causes a false sense of confidence - I am guilty of charging overnight and unattended in the stairwell. A Lipo pack you know presents a risk if badly treated so you look after it and watch over it making it just as safe in the long run IMHO.