I don't think any systematist or taxonomist will tell you that taxonomy is a simple endeavor. As in every biological science there are gray areas, areas where even experts disagree. And when new evidence is discovered and new techniques evolve, as in every branch of science, the taxonomy is re-evaluated. Just compare the two "official" checklists for the the U.S.: Collins' CNAH list and the SSAR (Crother) list to see how committees of experts can disagree on U.S. species. And, add to that, the debate on the very concept of "species". Differing philosophical approaches yield different answers and interpretations of evidence.