First of all, it's a high price tag for them to get top talent in the area. Waymo, Uber, Tesla, Ford (and a dozen other car companies), and who knows how many start ups are all vying for the same pool of engineers. Even if they get top talent, it's another story holding onto it. If you look at the LinkedIns (Anthony Levandowski, Jur van den Berg, Chris Thrun, all the founders of Otto and Argo AI), they change jobs every two years.
Their success I feel depends on how much control they are willing to give up in partnerships and how good they are at choosing winners in the race to autonomous vehicles. They already have $1B invested in Didi last year--it will be interesting to see where they go from here.