Poloniex is one of the greatest prizes in hacking at present. Lord knows how much money is stored on there. I'm sure they're extremely careful but another hack is only a matter of time.
It's already happened and somehow it'll happen again. Considering the returns on offer there's no way I'd bother myself.
They are being pretty careful about security though. Since the Cloudflare bug thing they've been manually approving large withdrawals, and there is a notice on their site asking people to enable 2FA.
If people are sensible and make sure that all their passwords are a) long and b) different on each site, plus enable 2FA they should be fine.