Buddy of mine bought a flip a few years back and the old owner had done exactly that, skipped the tack strip and just glued the carpet edge down to the slab. Looked fine for maybe a year. Then the glue let go in one corner and that carpet started creeping like a rug in a cartoon. He ended up pulling the whole room and doing it right, tack strip in strips of plywood anchored to the concrete. Cost him way more than if the guy had just done it correct the first time. So yeah, be scared, that shortcut always comes back around.