How safe is it to use online wallets? I want to know if it is safe to use online wallets? Can we trust online storages which stores our privkeys, passphase and wallet.dat files?

I read that at least one android wallet was hacked so I think this is not safe when private keys stored into foreign remote storage. Maybe I misunderstood something?
Holy war question in fact.

Yes, using online wallets means that you trust their developers. But the same situation is in standalone applications - you should trust their developers.

Many online wallets do not store your private keys. But it does not mean that they do not have an ability to steal/lose your funds