Arbitrum, as a layer-2 chain is completely safe as the platform is run by smart contracts and there is never an actual threat of bank run. You can easily withdraw any amount of deposited ETH without having to worry about clandestine lockins. The only concern is whether Arbitrum is sufficiently decentralized. Another aspect of Arbitrum's existence is that it still relies on Ethereum for its security, making it a relatively safe network, on its own.