I personally have never made any bad experience there, but at the same time of course I can't guarantee anything. In general exchanges are not 100 % save and can always be victims of hacker attacks.
Also people from certain countries can face problems if they send their money to US-based companies: read just this example (especially the comments).
What the fuck!
It was told me by my iranian friend, His brothers funds are still in the bittrex and he can't withdraw them.