Using the word "simple" probably wasn't the right word. When I listen to the guitar and the drum patterns, they're generally pretty average and not very exciting. The exciting part of the songs come from her vocal melodies and the keyboards/orchestration. My opinion that they're not good song writers comes from the fact that the drum and guitar patterns have seen very little variation over multiple albums. They seem to recycle a few riffs and rely on Sharon and the orchestras to gloss over it.
Digging: Sara Bareilles - The Blessed Unrest