  Optical dielectric metasurfaces composed of arrayed nanostructures are
expected to enable arbitrary spatial control of incident wavefronts with
subwavelength spatial resolution. For phase modulation, one often resorts to
two physical effects to implement a 2\pi-phase excursion. The first effect
relies on guidance by tall nanoscale pillars and the second one exploits
resonant confinement by nanoresonators with two degenerate Mie-resonances. The
first approach requires high aspect ratios, while the second one, known as
Huygens metasurfaces, is much flatter, and thus easier to manufacture. We
compare the two approaches, more focusing on conceptual rather than
technological issues, and identify fundamental limitations with the latter. We
explain the origin of the limitations based on general arguments, such as
reciprocity, multimode/monomode operation and symmetry breaking.
