On the other hand, the big downside is: they can become an incendiary bomb. Under certain conditions, they can instantly burst into a super-heated combustion that firefighters can't put out, as it keeps reigniting.
Hopefully, Musk's new "100 year" battery will eliminate this, as well as the controversy over mining the raw materials build it.
The "Million Mile Battery" is based on Nickel rather than Lithium.
Well sure, if you want to be all "let's look at the downside," there is that...
;-)