I have also been screwed by Bittrex. I opened one account without issue. I then opened a second account to do some trading with money that has to be kept separate and that account then got locked for using a duplicate ID. 

Their 'support' system is a joke - email only and nothing but an initial automated reply. No phone numbers on the site and I found one on the Internet that I tried and I just got someone's voicemail.

Regardless of 'growing pains', not being able to provide basic customer service makes me consider them not a legit business.  I'm going to follow up with the Secretary of State's office in Nevada with the hope of at least getting my frozen BTC out of Bittrex.

If you use them for trading I would move all funds off into wallets you control as soon as you complete a trade.

And I have also used Binance a bit as a few others have mentioned. No issues so far.