Never heard of this particular one, but the truth is you should never, ever use any unknown exchange to trade or store Bitcoins as it is only a matter of time until you lose them (usually get stolen). I personally lost a lot of Bitcoin this way, although forget the name of the exchange. I also lost a lot when Cryptsy closed.
Waves is not an unknown exchange, currently decentralized exchanges are not very popular but that could change, to be honest this seems to be FUD the op did indeed receive his coins on his wallet after some time, this seems more like a problem with the speed of confirmation than anything related to waves and their platform.
It's indeed not a Waves issue, but a gateway issue with fee & confirmation time.
Waves itself is decentralized, but their is no existing way to have a decentralized gateway (this for any coins that use IOU tokens).
Anyway, glad to hear the OP got the wBTC tokens... like the others ...