Keeping your money into any online wallet like Coinbase or Blockchain is risky. These online wallets are more prone to hacker attacks. I am not saying that your offline wallets are also safe. But offline or hardware wallets are safer than online wallets. However, coinbase always keeps your money into cold storage. Once you receive a transfer of bitcoins to your coinbase account, you will always see that the bitcoins are further transferred to another account which is their cold storage. This is an extra layer of security they provide along with the insurance cover. I have using their service since long time now, and I have never faced any issue till date.