They are the opposite of Decentralized Wallet...
Any wallet where you store your private keys on their server are never secure, you're pretty much trusting third party in holding your coins without any guarantee that they won't get hacked or any similar related incident in the future.

@OP
Are you going to hold your coins for a long time as a long term investment or are you gonna spend it daily?
For long time holding either Hardware Wallet if you have the money, or print your wallet on a paper and laminate it to avoid any damage on your paper.
If you're using it for spending get a Desktop Wallet, as long as you keep your PC clean and your wallet updated to latest version there won't be any trouble with your security.