1. Attacks on their network are too often cause the site unavailable or busy because of the attacks.
2. It's now a centralized exchange.
3. 2FA bypass security flaw. Some were saying even if you are 2FA enabled. You can still withdraw your funds.
Bittrex is good uprising exchange. I've been trading there for months but not a lot. But so far, I've never encountered any problems at that exchange. I would definitely chose Bittrex over Poloniex. Because it is operating very well. Bittrex just not added other good coins.
Bittrex is too annoying. They ask you for your real name, real place of birth, phone number, real address... if anything is wrong, your account gets rejected (they check against public records) and they will ask for an SMS confirmation message.
I don't want to trade against fiat, only against other cryptos. Why do I need to risk giving personal information to some altcoin exchange when I don't want to interact with the banking system? I just want to trade between fiats.
So I stick to Poloniex because they don't ask you for all these details. I just don't trust exchanges enough to give them my real data, who knows where that would end.
The banking system isn’t the issue. It’s the fact that you have to declare all income and losses for income tax purposes. No point in making a million dollars if you aren’t allowed to withdraw and spend. If you take a capital gains loss it’s nice to use that as a right off. Keep your paper trail for audits.