Summary for those in hurry: with NVIS, the ionosphere is your reflector, but you must use bloody low frequencies (e.g. 8 MHz in daytime, 4 MHz at night, because the ionosphere changes properties in sunlight). This will require loooong antennas (which may be horizontal, e.g. between trees) and the full wavelength is about tens of meters long, so you can’t really use full-wave antennas (quarter wave is practical) and all of this will limit the data rate considerably. Overcrowding of the narrow range of good frequencies may occur if it becomes popular.
Thanks for dropping the abbreviation. :) For those who want text:
https://en.wikipedia.org/wiki/Near_vertical_incidence_skywave
Summary for those in hurry: with NVIS, the ionosphere is your reflector, but you must use bloody low frequencies (e.g. 8 MHz in daytime, 4 MHz at night, because the ionosphere changes properties in sunlight). This will require loooong antennas (which may be horizontal, e.g. between trees) and the full wavelength is about tens of meters long, so you can’t really use full-wave antennas (quarter wave is practical) and all of this will limit the data rate considerably. Overcrowding of the narrow range of good frequencies may occur if it becomes popular.