I am installing solid horizontal bamboo strip flooring over a stable plywood subfloor that has a few low spots ( perimeter mostly and maybe 6-8 inches at joints ).

I was going to use 15# felt to build it up to flat but chickened out because the manufacturers installation guide said that the wooden subfloor had to be " filled, sanded or the flooring will fail ."

Any comments about this statement would be greatly appreciated. I would like to use the felt method because I dont feel good about having a patching/leveling compound under my flooring with a bunch of nails driven through it. ( cracking, loose compound etc. )

Thanks in advance.

First time I have ever heard that statement, regarding the sub-floor and product failure.
I believe that filling low areas, if not too deep; can successfully be raised using 3-4 layers of 15lb felt.
This should not create a failure of the floor. However, since I am not providing the warrantee on your floor, perhaps a call to the supplier is in order for clarification.