Should I be concerned? I started using Bittrex as my main exchange but now I keep hearing about accounts being locked away.. is Poloniex still good?
If you would've read the entire thread, you would've gotten your answer. @OP is from Ukraine, see
No, I'm from Ukraine.
and as previously stated somewhere on page #2,
Are you by any chance in the Crimean part of Ukraine or close to it?
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Comprehensively Embargoed Countries:
Crimea Region of Ukraine, Cuba, Iran, North Korea, Sudan, Syria
I think bittrex is legally obligated to do this, due to the relations US has with some of these countries &(it's residents).
So, unless you are from one of these countries, i don't think there is much to worry about in this case. It isn't MtGox.