In terms of safety, NiMH is, in my opinion, similar to LiPO4, and generally safer than LiIon. Rapid thermal events certainly tend to be more spectacular with LiIon than NiMH. NiMH are generally safe when punctured. read more
NiMh batteries should not be regarded as completely safe, although they are less volatile than lithium batteries. The negative electrode (if I recall correctly) is pyrophoric, meaning that if the battery is opened for whatever reason, it may ignite suddenly. The materials used in NiMh batteries are not as toxic as those in lithium batteries. read more
Weight: NiMH batteries are larger and heavier than Li-ion batteries. Power: Li-ion and NiMH batteries can actually hold a similar amount of power, but the lithium-ion cells can be charged and discharged more rapidly. read more