No, Poloniex is definitely
not safe. I have been one of the largest FoldingCoin recipients for the last 4 months, and my $3000+ FLDC deposits would get stuck for days. Their support was so bad, they had to take down the TrollBox because that's all it was used for: people inundating the Trollbox, begging and pleading and threatening because they needed help.
2 months later and the situation is worse. You'd think that because Poloniex is "a cryptocurrency exchange based in the U.S. with Corporate offices in Wilmington, Delaware" that you'd be able to trust them.
I went and visited their corporate offices. And this is what I found:
https://bitcointalk.org/index.php?topic=2142453.0