They are relatively expensive, they have fairly high self-discharge rates and aren’t very efficient. Last I checked, they had on the order of 1%/day discharge (some versions are as low as ~20%/month). They also are not very efficient in the charge/discharge cycle, losing 30-40% of the power put in. Lithium currently is around 10%.

Their primary use is in applications where their long lifetimes outweigh all other considerations.