I've worked at both Yahoo and Google. I've never worked with Marissa directly but I think your opinion of her is pretty much correct.
The biggest risk I see is that there is a huge difference in her role. At Google she stood atop a pyramid of other geniuses, with similar backgrounds and values, and was a filter for their ideas. At Yahoo she's dealing with a culture where engineering is not the highest value, and she's going to have to get off the top perch and descend into the ranks, clearing out the enemies of progress which exist at every level.
The biggest risk I see is that there is a huge difference in her role. At Google she stood atop a pyramid of other geniuses, with similar backgrounds and values, and was a filter for their ideas. At Yahoo she's dealing with a culture where engineering is not the highest value, and she's going to have to get off the top perch and descend into the ranks, clearing out the enemies of progress which exist at every level.