Thank you for your responses.
I'll use more the decentralized exchanges now. But most of the time they don't have high volume :/
well let's face it , these days with the current decentralized exchanges it's really hard to professionally trade but maybe in future we will see good ones with good volume and platform
your concerns about exchanges are legit , there is always risk when you leave your funds there and they may selectively scam some members like bittrex
but using them with cautious should minimize the risk