I think it depends on the exchanger site. Because I'm always leaving my altcoins in c-cex but it never lost. It would be better if you will leave your bitcoins or altcoins in some legit exchangers like c-cex and yobit to be sure that your bitcoin or altcoin would never be lost or get stolen maybe.
Is c-cex or yobit more legit than poloniex?
You have got to be joking right? Them two exchanges are scam exchanges and poloniex is the most trusted alt exchange.