Etherdelta was decentralized too. But it was hacked by replacing domain address. There is nothing safe in the internet at all) You can be hacked everywhere.
Yes they are all decentralized but the hacker was so intelligent to make it them easier to stole the exchanger profits.The more we improve they are also improve it's ability to hacked.
They are not decentralized and that's just a marketing gimmick. The bancor developer can't even maintain its vulnerability. Remember about it's raised around 200 millions dollars for nothing. It's not about the intelegency of hacker. it's about how the team can maintain its security properly.