Lithium-ion batteries are not dangerous in themselves, but it is important that you treat them carefully, because dropping a battery-powered tool can damage a battery cell. If you then connect the battery to a charger, the battery can become extremely hot and catch fire. These battery fires are particularly fierce and very difficult to extinguish. In the worst case, the battery can even explode and release toxic substances.
The problem is, is that you can’t tell from the outside that a battery has been damaged and you don’t always know if someone has dropped a tool.