"Not your keys, not your funds", that is usually the advise given to many newbies storing their assets on exchanges. Apparently, there are some mobile wallet providers that controls the private keys like FreeWallet. Using such wallet is just as bad as storing on exchanges because they can also hold your funds whenever they desire and there's nothing you can do but to comply with whatever requirements they need. Your assets may still be released but imagine the inconvenience and the lost opportunities.
It is preferred to use wallets where you control the private key.
I've posted a similar topic in another forum