it's as risky as using web wallets. If it's hacked, the hacker can simply edit the address linked to your user id and replace it with his address.
Also, there are some privacy concerns. As far as I can see, you can only link one address to your uid whilst it's advised to use a different addy for each transaction.
If you care about your privacy, you will have to login to your account and add new address whenever you expect to receive funds which is not practucal at all.