DDEX.io is operating on top of the 0x protocol which allows for wallet-to-wallet trading. While wallet-to-wallet trading sounds more decentralized (because you do not need to deposit your funds to a smart contract), it creates other problems.
Converting ETH to W-ETH
Firstly, to trade you have to convert your ETH to W-ETH (Wrapped ETH) which is an ERC20 token wrapper for Ethereum. Since Ethereum does not comply with the ERC20 protocol, an ERC20 token is used to convert your ETH into an ERC20 format. After you finish trading, you need to "Unwrap" your ETH (transforming it from W-ETH back to ETH)
Access to your private wallet
Because you are trading directly from your wallet, you have to grant access to your wallet when you are creating an order. You give access to a specific token and a particular amount, however, you must always remember to cancel your orders to avoid waking up one day with unrecognized transactions on your wallet.
DDEX takes the price difference!
When there is a sale order of 100 TOKENS @ 0.8 ETH each, and you place an order to buy 100 TOKENS @ 1 ETH each, you are paying 1 ETH, and the seller gets 0.8 ETH, the difference of 0.2 ETH goes to DDEX. They do promise to reimburse you the amount sometime in the future, but that is not a decentralized solution.
By contrast, EtherMium matching engine in the same situation will pay 0.8 ETH and take 0.8 ETH. Therefore it doesn't matter what price you set in your order; your order will always execute at the best available rate.