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Li-ion have several advantages over lead-acid or even NiHM. They are very adaptable to shape and are much lighter. They don't suffer from battery "memory" and have an extremely low self-discharge rate.
Their main disadvantage is they lose capacity based on age, not charging cycles. Li-ion batts lose about 20% charge capacity every year. Exposure to high temperatures shortens the lifespan even further. As they age the internal resistance also rises, reducing the maximum current that can be drawn.
Li-ion chemistry is not as safe as nickel metal hydride or nickel-cadmium, and a Li-ion cell requires several mandatory safety devices to be built in before it can be considered safe for use outside of a laboratory. These are: shut-down separator (for overtemperature), tear-away tab (for internal pressure), vent (pressure relief), and thermal interrupt (overcurrent/overcharging). The devices take away useful space inside the cells, and add an additional layer of unreliability. Typically, their action is to permanently and irreversibly disable the cell.
So, while it seems they would be ideal in an aviation environment, due to reduced weight and space conformity ability, they look like poor long term choices for things like electric cars unless the manufacturers are able to tackle the longevity problem.
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