It is just as secure as the base chain (unlike polygon) and has low fees and has been live for the past few months. This is a perfect solution to simple payments.

The difference between optimistic rollups and zero knowledge rollups is that you can’t deploy arbitrary smart contracts to zk rollup, it only supports a limited set of use cases, such as simple payments for now. Read more here https://vitalik.ca/general/2021/01/05/rollup.html