[fbrusch]

Sidechains have different security properties. You probably know xdai (https://www.xdaichain.com/), where you can transact tokens that "wrap" dais from Ethereum mainnet. The sidechain is secured by a proof of authority consensus, where you require a fraction of validators to behave honestly. If a certain number of validators collude, they can do nasty things, such as censoring transactions from a certain address, or keeping the chain from making progress. This is not possible in a rollup, either optimistic or zk, because any single user can always commit a batch or report fraud on L1, so you just need a single honest actor for things to work.