Yes, crashes,that will be an important factor and brings in the luck aspect. The T de F seems to have too many crashes,particularly in the first week, compared with the other GTs.
Much will depend on the run-in and the final km of the "bunch sprint" stages.
I think Sagan will easily get the Green in the T de F. Even if many stages will have bunch sprints for the likes of Cav, Greipel, Kittel and co.,Sagan will still be in the top 6 or so. He will also get points in the hilly stages where the top speed sprinters go out of the back.