Using BC5 you have to compute the vector from the normal map like that for each pixel
A fairly trivial calculation by today's standard. The advantage is that the R and G channels are completely uncorrelated, and you have 8bpp divided between just two channels instead of 3. If anyone happens to know of a in depth comparison of BC5 and BC7 specifically for normal maps, I'd be interested to see it.
Does that mean BC7 can't be used because of that or it's a particular case where BC7 should not be used ?
I would say to use BC1 over BC7 whenever you can get away with it, e.g. whenever the quality difference will not be noticeable.