that's not a risk, brainwallets are easy to crack because the passphrase is the entropy of the seed, and we are not a very good source of entropy. The mnemonic phrase works the other way round : the words encode the entropy of the seed, which is chosen by a proper random generator.
not being across the limitation of brainwallets I found that article illuminating and rather disturbing. thanks for the explanation on the differences. i can rest easy again