If you store coins in online wallets and if the site disappears in a sudden one day, you will lose everything.
Are you scared?
When you want to store online wallet, You have know what kind wallet there, private key and seed it can be use for any wallet, like MEW, when you have private key it can be use for any kind like metamask, cryptostore. it shouldn't scare when your private key can use any kind of wallet when online wallet your stored dissapear.