Years ago I read a complaint that they promise unlimited withdrawals, but in reality only allow $50k a year. When I recently checked their site it says they allow $100+ million a year for their top tier. However I haven't had a chance to ask anyone about their experience of a really big withdrawal yet.
I don't really get why exchanges would have any limits. They usually don't have incoming limits. You can send any amount of coin to them, you can also send any amount of fiat to them.
If someone sent a thousand coins to the exchange, and if someone else bought those coins (or more than one person), those sent in the equivalent fiat, why should the first one not be allowed to withdraw it all? The exchange has already made their profit on trading fees or any other fees they may have.