NiCd and NiMH batteries die when left in slow drain devices as the first cell to go empty starts to be reverse charged. These die and often leak pretty fast and you see that all the time if you repair old devices.
Li packs don’t go empty as the battery protection circuit cuts the slow drain when they reach low water voltage. They are revived when the protection mode charge reaches low water mark again. They’ll be fine unless you leave them for years and years. Even then they generally never leak.
That isn’t really true though, any vampire currents or leaving it only partially charged can leave it in a state where it reaches the cutoff voltage when you aren’t using it. Then self-discharge takes it under the limit of the charging circuit and the device never works again.
With a device that can run off AA or AAA cells, you can use NiMH (NiCd has zero place today) and remove the batteries when you are done and put them back in the pool of other cells you use. By the nature of Li-ion, as soon as that cell is made it has a limited lifetime until it no longer works just due to reactions that happen within the cell, which means any infrequently used item that might last for decades otherwise now has a lifetime of maybe 5-10 years.
