Answer seems pretty clear to me:
"The upshot? Use a simple password, which many people have done, and it's easily cracked despite the appearance of complexity.
Castellucci and his co-authors checked a trillion passwords for $55.86 and recovered 18,000 wallets.
Ryan Castellucci of White Ops presented some of his research into this area last August, and is the common link between two new multi-author papersone out this week and the other available and being presented in two weeksthat dive even deeper into the problems with brain-wallet protection and the techniques which bad guys have used to empty such wallets. Also last August, he released Brainflayer, a tool for automatically testing passwords against brain-wallet encryption keys."
Since you've stored the private keys inside the blockchain, you wouldn't need a transaction in order to brute force it.