Online wallets are not as secured as it stored the wallets information with a third party and that information are easily accessible to them. Making it easier for the hackers to steal all your funds through hacking into the online wallets site and get all the stored information. Whereas hard wallets (Offline Wallets) do not have that, instead all the private information along with the keys and the stuff required to access the wallet is with the owner himself so that is why hard wallets are more secure and should be chosen over online wallets.