Is placing a limit order safe? By any chance that others might detect this order info and attack? (For example, Sandwich attack?）

Currently, we are using the broker for a limit order, which is a relatively centralized way. It’s less safe than our market order. It could be attacked by the sandwich attack if both the broker’s private key and the broker’s signature are stolen. In this case, users will get a worse deal than he could have got. 

However, in defi space, there are two ways to implement limit order. 1. on-chain limit order 2. broker model like we do. Both ways actually have the same level regarding safety.