The inserts are not actually held in place by the adhesive. I just added this (like Loctite) to keep them from backing out. The inserts are about 1/2" long with the inside threaded for a machine screw or bolt; the outside has large diameter wood screw type threads to lock into the wood.
This is admittedly not as strong as through-bolting but is much stronger than a wood or sheet metal screw alone. The proper way to attach the rails, I believe, is with through bolts from above and wood bungs to cover the holes. But if you have to fasten them from below (or want to in order to avoid digging out the bungs in order to remove them) I think the inserts are a great alternative to sheet metal or wood screws.