Why is it dangerous to trust `block.timestamp` for very important time-based actions in a smart contract, especially for short times?

block.timestamp is a variable that represents the Unix epoch timestamp of the current block, set by the block's producer (miner in Proof-of-Work, validator in Proof-of-Stake). Trusting block.timestamp for very important time-based actions, especially for short durations, is dangerous due to two primary reasons: block producer manipulability and inherent granularity.