Why so many peoples choose online or hard wallets? I use Polo and Bittrex, they are more safety.
You have to trust a company, and companies can bankrupt (so you loose your coins)
With Polo and Bittrex the chance is small, but never say never. Just look how many respectable banks were bankrupt in the past
(Lehman Brothers, Fortis Fannie Mae and Freddie Mac ABN-AMRO etc.
Some where saved by governments, but don't expect a government will save a Crypto exchange)
Anyway, you better trust your own wallet.