i've used that website before, but how safe is it actually ?
It's safe. I've been using my address fro that site for almost 2 years and never had any problem plus it is being recommended by sites like coinbase and even blockchain.info itself if you will look for a tutorial on how to make a paper wallet.
It is safe, but if you are using that website directly, then you're not doing it in a safe way.
That website uses client-side javascript to generate the wallet. That means that you can use it offline as well.
The best practice is to use their code on a laptop/pc that has never been connected to the internet.
You can grab the code here and run it yourself with a webbrowser:
https://github.com/pointbiz/bitaddress.org