ALL wallets are vulnerable to hacking in different ways if they are not setup and used correctly..

It sounds like you are talking about a web wallet, a wallet on a website.
These are very vulnerable to hacking just the same as exchanges are. They can be hacked by hacking the website itself, ot hacking your computer to get your login details, or phishing you..
They definately are NOT unhackable..

Then you have desktop and mobile wallets..
Besides possible volnerabilities in their code they are only as safe as your device.. So if they can hack your device they can get your coins..
Even if you encrypt the wallet they could still get you with a keylogger or something..
Your device's security is in your hands..

Then you have "cold" wallets.. They can be quite unhackable if done right but you have to do them right..
If your seed or priv key are saved on a machine that the internet can touch, then they are hackable..
If you create them on a machine that was possibly infected or connected to the internet, they are hackable..
They are only unhackable if done right..

Then their are hardware wallets like the ledger and trezor..
Unhackable you might say right?
Well probably if you save your seed offline like write it down on paper, but if you save your seed in a file on a computer that is connected to the internet they can getcha....
Also, a lot of people STILL get hacked using these by trying to claim forks with scam wallets!!!
So you still have to watch what you do..

My choice is the Ledger hardware wallet but I am still very suspicious of the chrome app, and it needing "updates" all the time spooks me..
Seed written on physical paper..