When I took my NRA instructor courses, I was in the minority as a civilian with no military or law enforcement background taking the classes. The rest were mostly military or soon to be former military instructors who were taking the NRA courses to transfer their military credentials to a civilian credentialing.
Here in Texas, to become a state certified CHL instructor, you must be a certified instructor first before they will allow you to become certified as a CHL instructor. Texas accepts NRA, military and law enforcement instructor certification to satisfy that requirement. So, check with each state that you may instruct in for their requirements.
I agree with most of what the other posters have stated. The only thing that I would add is that for many first time students (many of our students have never handled a firearm), the NRA courses are a great way to get started. Yes, they are basic; but cost is very reasonable and are offered around the country, usually not far from where they live. These courses are designed to reach the larger audience and the training material works. But you must always remember, they are teaching the BASIC fundamentals upon which you build the advanced skills.
At the end of each course the students are encouraged to seek further training. I know that in our courses we emphasize that and try to impress upon them the importance of higher level training and continual practice before venturing out into the real world with their firearms thinking that they can handle any stressful situation.