I'm honestly not sure what you're referring to by saying that they gather orders on private servers but anyhow, do you have a proof of your claims? They still give you full control over your funds, and I think with all the hacks that are going on in the crypto scene, that was counts the most. You can check
this list for decentralized exchanges.
https://www.reddit.com/r/EtherDelta/comments/724qsn/wheres_etherdelta_order_book_resides/One of the moderators of Reddit says 'There is an off-chain order book server.'
I am accepting that this is less scary than centralized exchanges but off-chain order server means this is not exactly decentralize exchange.
But of-course most of this Decentralized exchanges run on a smart-contract which has been created by someone so the have ability to control anything. In my opinion it's totally impossible to have an exchange that is completely decentralized away that the buyer and a seller deal together without any central authority.
So you either accept the semi decentralized exchanges or you go with fully centralized ones
But the contract creator can limit their authorisation on smart contract so actually the contract has not a owner.
I do not think it is really focusing on the "decentralized" part because of the data collected and how it works to be honest with you.
The point of decentralized exchange is not the data that is stored and where it is stored but more about how there is no party that is holding your coins some place you do not want while you trade, binance for example is the biggest exchange and what do you do when you want to exchange your coins into other coins ?
You give your coins to binance and they hold it for you and you trade your coins, so basically person A gives bitcoin to binance and Person B gives ethereum to binance and binance changes the numbers and now Person A has ethereum and Person B has bitcoin but in reality all those coins are at binance not at the traders. Decentralized is trying to work on getting rid of that hopefully.
Okay, Ether Delta is less scary than centralized exchanges because you can get back your coins whenever you want. And surely your coins are yours, there are no one to trust. On Ether Delta you are trusting to the code I mean Smart Contract. But here, the problem is orders can manipulated. Your order may remove by the owner of Ether Delta. Okay this is not a huge problem. But it means, orders can manipulated and Ether Delta is not an decentralized exchange.