Lipo batteries are not safe-chemistry. If they go into thermal runaway (venting), they vent hot gas, flames, and may actually explode. The only thing protecting them in a mod is the circuit protection of the processor.
IMR batteries are safer chemistry. They may still vent gas in thermal runaway, but they are not likely to vent flames or explose.
I will absolutely +1 that. I know mods like the variant and the vaporshark use them however they are volatile pieces of equipment and I don't feel safe using them!