The fact that the hold downs don't work may be for a few reasons. The first may be that you're doing them wrong. That usually is what's happening when something doesn't work for me. ;^)
These techniques may also not be for holding someone down at all costs. You need to see them in context. As aikido comes from samurai battle field arts, these techniques make a perfect opening for a final strike with a sword, staff, or spear in a vital area (the back of the neck, behind the ear, for example).
Are there any police officers on the list who have used an aikido pin on an arrestee? Has it worked without breaking a joint?