Cryptocurrency exchanges not recommended to use. Fraud crypto exchanges list, scam crypto exchanges. Any centralized exchange can block your funds, but these are seen in the regularity of assigning user funds, possible scam, fraud exchanges, not honest exchanges and exchanges with problems. They are running an undercover money grab.
Bittrex - block account without notifications and not possible to complete KYC if you will try
GateIO - freeze accounts with funds for no reasons
HitBTC - HitBTC is known for holding your money hostage and coming up with delays/excuses over and over until you give up. Stay away!