system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are flywheels still unworkable? It seems to me that they're such a simple method of power storage, but every article I read seems to be about how they're out of reach. What's the problem?
gpt
Efficiency, for one.  Flywheels are used in a few applications, like very large online UPSes (uninterruptable power supplies). High speed flywheels can make a big mess and kill if they come apart, so some work is being done to find good designs that can run at very high RPMs.  This helps make them more efficient and higher capacity, while making them smaller in size. 

EDIT: typos EDIT 2: and size. Size could be the biggest reason.