I disagree. There are lots of videos showing guys with backpacks carrying bails of weed worth a few thousands. So carrying heroin instead of weed worth millions would be soooo easy and it does happen. And just because of the fact that they do catch 10% in open border areas, I am sure a lot of it gets through.  We just don't know what we don't know. One way to find out is build the wall and force everyone to the entry points. If after we have a wall seizures go up you will have the answer.