The only problem I faced on Poloniex, is the inability to place an order at peak times. In general, I do not have problems.
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.