I have never used them from from looking at trust pilot reviews, it turns out that when they started around 2014 they used to be great in terms or their services, deposit and withdraws but recently the reviews about them falls between a shitty service and scam. A lot of their users are complaining about being unable to withdraw their funds after a period of time. They are disappointed, frustrated and have regrets.
Here is their review - https://www.trustpilot.com/review/poloniex.com

To avoid these issues just stick with a trusted crypto exchanges use mainly the top four or five but with extreme caution. Check coinmarketcap for their rankings.