It is not a scam and it is not decentralized fully. It runs on top of bitshares network which means that when you create an account you can use it with other providers on blockcshares such as rudex and cryptobridge. How they work is that you deposit money to their exchange and then they will issue their own version of token on bitshares network so you trade that token for other tokens to trade crypto. Every token is or at least should be becaked up by 1:1 ratio of the real crypto. I have traded few cryptos with bitshares network via either openledger or cryptobridge (tho they use same network just you can view different tokens on each and deposits on one mean you trust that rpovider). So it acts as a gateway or central authority on bitshares. They could have some problems here and there as I had problems with my withdrawls once but simply ticket got my problem solved within cuple of days.
This is their main website:
https://openledger.info/ where you can check information
or yet go to their bitcointalk theard :
https://bitcointalk.org/index.php?topic=1390255.0