I sold some Ardor, sent the bitcoin to cryptopia, about 4 hours ago. no issues. Cryptopia wasn't showing the bitcoin right away so I took a 4 hour nap, woke up, funds were there. My solution to this problem has been to simply not to try withdrawing altcoins unless the site seems to be issue free by doing a casual search. Usually I have just kept the coins on the exchange and they're still around. I make it a point to try to only withdraw bitcoin.
If you can avoid having to deal with customer service in the first place, that's the best way to go about it. Unfortunately for you you didn't.
I'm not saying you did anything wrong, but a lot of times when people have issues like this it's because they did something wrong, like in the case of NEM not copying and including the proper "message" when sending the coin, or filling out all the fields properly. These mistakes can be easily avoided with a little care. They don't hold your hand in the crypto space so it's important to take every precaution.
The main disadvantage of Poloniex is they're riding on their success, not actively improving. Their withdraw limit of $25,000 is really quite meager by today's standards also. Seems like a lot of exchanges lack attention to detail, blaming it on customer load. I can tell you if I was running my own exchange I'd be constantly looking for wallet updates and reviewing customer issues as best I can. If as the owner I had a 3 month backlog of complaints, to me that would be unacceptable. I'd stay up day and night until they were addressed and hire the needed people. With the money these places bring in from transaction fees you can't tell me they're hurting for money.