All of your keys (wallet addresses) are encrypted with your password. So the stronger the password, the better the protection. Other than that, the database never stores your info in cleartext so it’s very safe. Just make sure you use a good password and, if you are comfortable with the technology, use 2FA (two factor authentication).
In the very unlikely scenario that the web-wallet gets hacked, the hacker won’t be able to make use of the encrypted keys if you set a proper password
In order to benefit from using web-wallet, follow these steps:
1 Use a complex password
2 Make sure you enable 2FA for your phone, save 2FA code and keep it somewhere safe in case you loose your phone
3 For the sake of emergency, safely export and store your private keys in a secure place (maybe archived with password somewhere offline)
If you feel more confident in using WebWallets instead of PC wallets due to your PC not being secure or subject to multi-user use increasing the risk of Malware and Viruses etc please consider using the WebWallet or Android Wallet, or if you prefer a Cold Wallet one that is not connected to the network, you can use a Paper Wallet.