It's not just unstable, it's a security issue because control over CPU voltage allows someone to use that instability to compromise computations that are supposed to be performed securely (e.g. look up "Plundervolt").
If plundervolt is a viable attack you’ve got much bigger problems (the attacker must already have gained full privileges). Not sure how this is relevant to just using throttled or similar to conditionally undervolt the CPU with fixed levels that can be proven reasonably stable.