Anyone can also work on cracking a private key from anywhere too without the wallet file.
Bitcoin private keys have 160 bits of security, maximum.
So, you just have to make sure your brain wallet has enough
bits of security (128-bit or more).
Sure, but if you have a 160 bit brainwallet then what is that, 14 - 15 words? You'll have to write it down. So that's a paper wallet, where you're encoding the private key with words instead of base58. It's not a brainwallet.