While it usually works as advertised (sort of, after the FMC SPEED power addition on the FMA, soon followed by DRAG REQUIRED on the FMC scratch pad) it'll occasionally attempt to leave you hundreds of feet high at a hard altitude or fly you through an altitude on an arrival when it sees it can't make both of the next two restrictions. TK will probably tell you that can't happen, just leave it in VNAV PATH. As has been said, "trust but verify" in VNAV.