In most cases surgery restores the stability of the ACL injured knee. However, surgery may not be successful and some of the reasons for this are listed below. I hope this encourages people to follow through with their surgeon's and therapist's recommendations. 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.