It seems a little clarification regarding ND filtration is needed. ND filters simply darken exposure. They are primarily used in cinematic and still photo applications where the camera operator wants a shallow depth of field, but current shutter speed and aperture options won't allow it.
If your standard, bright sunlight exposure is 100 ISO, 1/100 second shutter and f/16 aperture, then pretty much everything in the frame will be relatively in focus (sharp). By introducing ND filtration, you effectively lower the ISO by reducing the amount of light that enters the lens. This allows you to "open up" the aperture (f/5.6, f/4, or lower, depending on the filter factor or ND rating. With a larger aperture (smaller f/ stop number) your depth of field becomes more shallow, throwing the foreground and/or background out of focus (what folks here are referring to as "blur", I think) thereby emphasizing your subject.
The only way to get "motion blur" with an ND filter is by changing the shutter speed, instead of the aperture, which is a mistake because then you have poor definition, or resolution, overall, but especially if the camera, the subject, or both are moving. Which, in the case of most drone work, is 99% of the time. If you want crisp motion then keep your shutter speed as fast as possible while balancing exposure (ND or no ND).
Of course, shooting 720P has less resolution than Full HD, which has much less resolution than 4K, but that is a whole other discussion.
I hope this clears it up a bit.
