First time poster here. I'm a robot noob but I have worked in the business of lithium batteries for a decade or so now.

As others have said here's no fundamental difference between lipo and (what are commonly referred to as) Li-ion batteries, it's just packaging. All Li-ion cells are a carbon anode with a cathode composed of oxides of cobalt, nickel, manganese or a mixture thereof. Lipos are just Li-ion cells in a soft plastic/aluminium laminated encapsulation, sometimes with a gel to stabilise the electrolyte.

All lithium-ion cells can go into thermal runaway, which is a very rapid exothermic reaction leading to high temperatures (>1000°C possible) and lots of gas. The gas is poisonous and they can produce a lot of heat and flames but as long as you keep your distance and ventilate there's no real danger.

The cylindrical cells you get in a drill or other consumer item are in solid steel cans, which means that rather than just releasing smoke as lipos do (because they pop like balloons at low pressure), the gas builds up inside until they explode. There are safety vents but they can't usually cope with the rate of gas generation and the can ruptures. A bag will not help you here. The word explode is not an exaggeration. Example:
The initial "event" there is probably just one cell, I would guess an 18650.

It's true that NMC Li-ion are generally harder to get into TR than pure cobalt, but they still go eventually. They just need to get a little bit hotter. Severe damage would be enough to set one cell off, and the heat would then ignite the rest of the pack.

Overall it would be very hard to "prove" NMC batteries to be safe. Partly because it would involve some potentially dangerous testing, partly because it can cover so many things - different cell sizes, can designs, actually completely different chemistry (different proportions of Ni, Mn and Co).

(This is a good resource on battery types: http://batteryuniversity.com/learn/a...of_lithium_ion )