It's not that the website (Brainwallettx.github.io) is vulnerable but it's simply that people are in most cases not able to create enough randomness. It's very unlikely that you will make a passphrase that no one has used it before, and the risk lays there. You can create an Electrum wallet offline and simply memorize the seed, that's far more secure.

As for Bitaddress, then the website is fine as long as you're generating the wallet offline in a LiveCD linux distribution and If you want to generate SW addresses you can use SegwitAddress.org