Despite the advances of surgery, ACL is still a major injury.
Few issues here:
-Physical damage
-Mental recovery
-Proprioception
Physical aspect is obvious - you tear your ACL, you gotta have it repaired via the latest Orthopaedic method, as well as checking for any associated injuries usually to the cartilage.
Proprioception - we've all got tiny tiny "sensors" in our bodies detecting certain things (i.e touch, heat, pain etc..). Proprioceptors are those responsible for balance by providing info to the brain about your joint / muscle length / angle etc.. With major injuries (even twisting your ankle, im sure we've all done that), these get fecked. How many times have you had an injury, like the ankle twist, and then realise that similar injuries start occurring in the weeks after? Why do certain players become "injury prone" when they werent prior?
Its because their ability to maintain balance / posture / composure diminishes with certain injuries, and building proprioception back up is definitely doable which is why you have examples of players who reach their peak again, but like a lot people mentioned, this is down almost 100% to rehab and physiotherapy. And im talking days and weeks and months of sitting on your ass, extending your leg up, then moving it up and down for like a bloody hour. Boring and tedious as shit, but those who stick with it reap the benefits. Having said that, the degree of proprioception required could be dependent on sport intensity (PL vs Sunday league).
Mental Recovery - probably the biggest obstacle to most people. Mental strength. The sheer fear of "the injury happening again" can either cause you to put safety first and thus play to a maximum of 70 - 80% of your capability, or lack of trust in yourself / medical staff / physiotherapist can you make you feel that the injury never healed, and you're "not back at 100%". Of course you're not at 100%, you've been out for friggin 8 months, give it time and trust your body. Some people DO require counselling on such matters, and many patients do opt for that to keep their mental state in check.
ACL - injury itself is just another injury, its the post injury recovery period thats a test of ones character and physical state.