I have been using poloniex for more than 1 year now, till now I didn't face any problem with this exchange.I have many bitcoins in poloniex exchange where I am trading happily without the problem and when it comes to withdrawal and deposit they will pay on time without any delay, only if they have any issues with their software.
I am also very pleased with this exchange, there are simply situations where you can see that the servers are clearly overloaded.
The real scam is when the exchange crashes, yet there is clearly activity in the background (via cryptowat.ch or other API plugs) Yet you can't trade on their GUI.
This happened with XMR and most recently with STR. I'm done with them. My coins are off their site. They'll Gox soon.