Comment by runeks

With proof-of-stake, an attacker compromising a majority of private keys results in the attacker taking control of the blockchain. The attacker could publish only empty blocks using this stake majority, and there'd be no way to distinguish these malicious valid blocks from actually useful blocks.

This makes proof-of-stake inherently less secure than proof-of-work because for PoW blockchains, consensus is not affected by compromising private keys.