Secondly, some wallets are automatically backing up private keys to the cloud. Keys stored online are potentially a serious security risk. Make sure your apps aren't doing this, or if they have, move your funds to new addresses immediately.
It's generally good practice to only keep small amounts on portable devices in the event of physical loss or theft anyway. Larger sums should be locked away in cold storage, offline, somewhere secure.
I didn't know that the private keys are hosted on the cloud for the greenaddress app, I think it is dangerous if their cloud gets hacked so the hacker can spends our bitcoin without getting noticed. I prefer more to store small amounts on mobile if we travel or we need to, but the cold wallet should be offline.
Smartphones are certainly not designed to be bulletproof against attacks since the OS is not really suitable for storing sensitive information. Phone wallets are good for paying conveniently while you're outside. Just be sure to treat the wallet as a day to day wallet and not store majority of your coins there.