The graft doesn't heal and people end up with as much instability as when they started. People reinjure their knees. If you can damage a normal knee, you can damage a reconstructed knee. Also, there is a long period where the knee is much weaker than the opposite side after surgery. There may be more of a chance of injury during this period.
People end up with stiff knees either in bending or straightening.
The worst situation I have seen is where a person could not bend his knee enough. He had two years of problems and 3 surgeries to remove scar tissue. This type of problem is more common in reconstructions that are done soon after an injury. Straightening the knee is very important to walking. There have been people that cannot walk properly without pain on every step because they cannot extent their knee.