TLDR: Variations of common quotes are not safe. "Unique" quotes from notable memories may not be unique after all.
I have several brain wallets. They all are keyed by phrases that stuck out to me at some point in my life.
The ones that haven't been hacked are things I said (full sentence long original quotes including punctuation). Due to the context of the quote, they include word combinations that I don't think would ever come up again. They are "brand new sentences".
The one that was hacked was a phrase that I thought was unique to my life but turns out it wasn't. It was something someone told me when I was 4 that made an impression on me. I didn't check before I used it that it was a common phrase. I trusted someone else's "brand new sentence" and it wasn't brand new at all. The exact quote isn't on google but a variation of it was. I lost less than $100 but it's scary to think what if I had used that one on a big wallet.