In the proper device that is functioning correctly, not very dangerous.

But if something isn't working right, very.

They don't have automatic kill switches that stop the battery from charging. So they can be overcharged if there is not a regulator stopping it. That can lead to fires.

And if they discharge too fast and become hot they tend to get a little 'splodey'. Again, a regulator on the circuit is supposed to prevent that.

But if you put a LiPo battery in a device designed for a Li-ion that doesn't have those integrated safety features, it's basically asking for a fire.