#1: An exchange is not a wallet (although you could indeed leave your BTS there).

#2: Poloniex is worse than Bittrex, way worse, in my opinion of course.

#3: Risk levels are rising across the board when referring to centralized exchanges - USDT has nothing to back it and I think it's going to cause some bankruptcies and/or bail-ins - and I recommend getting off all of them by taking coins to private wallets.

Not to mention their risk of being hacked and making customers pick up the bill (another bail-in). https://www.bloomberg.com/news/articles/2016-08-07/hacked-bitcoin-exchange-users-to-lose-36-will-receive-tokens