I don't know enough about her history but found this on last year's Breeders Cup site. Unless they misspoke, she has won on dirt tracks.

"Strengths: Massive filly can make her own race, not needing pace help, and finishes with a flourish like few before her. She's a picture of perfection over varying racing surfaces and circuits, winning on dirt and all-weather tracks. Has exhibited the ability to put away her competition and win with ease where expected. Zenyatta owns a clear-cut victory over reigning Champion Mare Ginger Punch on a neutral track at Oaklawn Park.

"Weaknesses: Immense talent has scared many challengers away, so she's not well-versed against a big field of top-class rivals. Given her late-running style, she could be challenged if a big field showed up at the Grade 1 level; though she did rally to beat 11 sprinters in her maiden debut. Simply put, there are few flaws."

http://www.breederscup.com/bio.aspx?id=30441

One thing I consider against her: her age. At 6 she is in her prime but she is a good three years older than most of the up and coming racers. She wasn't in the Triple Crown competitions; she didn't even start racing until the end of the year she turned 3.

One of the amazing things about her is her size. I have seen 17 hand geldings before but never a 17 and a half hand Thoroughbred mare. When she bursts into gear, her stride eats up the track.