I know many XRB (Raiblocks) and other traders were unable to withdraw their coins for over a week. I took the hint, and traded for Litecoin before I attempted to withdraw. They held my Litecoin withdrawal for over 3 days before releasing the coins.
The company is registered in the U.K., but the sole owner and "director" is located in Costa Rica. The same guy owns a whole bunch of shady looking investment companies that he is the "director" of. The whole exchange gave me the heebie jeebies, but dammit, I had to have my XRB! (But seriously, it felt like it could have been run by Jeremy Jamm from "Parks and Recreation".)
Can't say I am sorry in the end, as I DID get my money eventually, and the trade worked out in my favor, but I will not be going back there until I have seen better reports in the troll box. When half the messages in the trollbox are "I can't access my coins!" for over a week, you know you have a stinker. Beware!
Mercatox is scam. You can deposit coins, can trade but never be able to withdraw them back. They never reply back. I think I lost 0.1 btc there while transferring eltcoins to my ethereum wallet, because of them. In UI, always says: "processing", nothing happens.