In my opinion,My Etherwallet is secure as long as you know your private keys.And it will never be hacked by soneone ad long as you do not post it publicly or in any way giving it and sharing it to others.It will really be hacked.
It's true. Since Myetherwallet never stored any private key in their database, an user does have full responsibility for securing his private key. So, if an address is hacked, it is the user's carelessness itself.