I haven't had any problems with Poloniex, myself.
Other than when they've 'temporarily' disabled withdrawals/deposits for certain coins like STEEM (which at this moment in time is disabled yet again!)
But I've heard stories that a lot of people have had problems.
Poloniex say this is because they've been overwhelmed with the huge influx of new users and have not had the resources to do all the required AML/KYC checks in a timely fashion.

I have been using Bittrex a lot more now (for both of the above reasons), have never had any problems with them, that would be my recommenedation -- unless, you want to trade very large sums, in which case Poloniex may be better because it currently has higher liquidity on many assets.