Just read that generating random a random number via
keccak256 is insecure. What is insecure about it and what are the implications of insecurity?
It's not the keccak, which is btw. just an hashing algorithm, but more the input you give to that, that can be cheated on. For example if you would try to make randomness with the block number, the miners could hack it. If you hash the block number with keccak, this would not change the fact that it's manipulable.