The vent in the link above does allow for more airflow as it is taller that any of the competition shown. I installed MaxxairII vent covers several years back and for one they do not sit flat on the roof. They provide ample airflow for our powered vent fans and as such we don't see a need to replace unless they deteriorate.
Another thing noted in the video is that the bug screen was installed in the competing vents but the ultra vent did not have it installed as they were very careful to only show the sides when placing over fan. Bug screen is sold separately and will diminish airflow a bit when installed. Enjoy your new vent covers.