What makes pollen varieties useful index fossils?
Depends upon the species. If the pollen spores are large enough, and of wide spatial distribution, and easily recognizable, and preserves well in sediments, and of distinctive age range (geologically), then it could be used. 

There have been mistakes in the past where modern pollen collected in microscopic pores in older rock, giving the illusion of rock ages much younger than they really are, so precautions against sample contamination are of paramount importance.