Surgery, overall, is not that technically demanding - by which I mean, most people are capable of being a surgeon with appropriate training and dedication. Surgery, of all sorts, is at least 75% cognitive - who to operate on, what operation to do, how to deal with variations of anatomy and pathology, and how to deal with unforeseen troubles and complications. Within all the surgical specialties I know reasonably well (general surgery, orthopaedic surgery, plastic surgery, vascular surgery, urology, head and neck surgery, neurosurgery) there are straight-forward procedures and more technically challenging ones.
For your examples, Achilles tendon repairs are not very challenging (and, also, the nearly overwhelming evidence suggests that nonoperative treatment is equivalent to operative treatment), while an ACL reconstruction requires experience and expertise in tunnel creation and graft tensioning to maximize outcome. I wouldn't be surprised if some rural surgeons or surgeons with less fragmented medical systems do Achilles tendon repairs (again, I'm not saying these should be repaired). Most ACL reconstructions, however, are done using arthroscopy and require tendon harvesting and/or preparing as well as bone drilling and suspending that general surgeons are not exposed to during training. Also, in the US at least, a general surgeon would probably not be able to get hospital privileges to do an ACL reconstruction - it is outside the scope of their training and expertise. Simple ankle/wrist fractures are easy, acetabular fractures and busted tibial plateau/pilon fractures are hard. While straight forward primary knee and hip replacements are something most people can learn to do reasonably well, revision joint replacements with bone loss can be quite difficult.
Likewise for the other specialties - most appendectomies are straight-forward, and there are even programs where family medicine doctors learn to do these as well as simple hernias, in addition to c-sections. A big recurrent ventral hernia, or a gastric bypass operation, in contrast, are challenging technically. For plastic surgery (and for orthopaedic hand surgeons), microscopic free tissue transfers and finger replantations are technically challenging, while split thickness skin grafting is not. And so on. Good news for trainees - I'd say pick a field based on your passion for it, the patients, and the pathology. It is unlikely you can't operate - but you may decide you are better suited to get really good at a small number of straight-forward operations, or you may decide you prefer complex technically challenging operations... these exist in all surgical fields.
Whoever you overheard was making an over-generalization. Hard and easy surgeries exist for all, but overall surgery is something most people can learn to do if willing to spend the time and effort.
Part of your question was about specialty boundary formation - why do orthopaedists do Achilles tendon repairs and not general surgeons? Why is spine split between orthopaedics and neurosurgeons? It might surprise you that general surgeons used to take care of fractures in the United States, and that in some countries there are trauma specialists who care for fractures as well as ruptured spleens and traumatic bowel injuries. This is an interesting topic, and specialty boundaries are always in flux. 75 years ago there was no arthroscopy, no joint replacement, and a lot of trauma was done by general surgeons... orthopaedics has changed a lot. And it probably will in the future too. It isn't just as simple as "Well, knowledge has increased so people have to specialize more and more" - that explains specialization overall, but not how it happens, who ends up doing something instead of someone else, etc.
If you are really interested, there are several books on it. Here are a few:
- For physical medicine and rehabilitation, which has cut away a reasonably large part of what orthopaedists used to do, see:
Gritzer and Arluke, The making of rehabilitation
- For a medical specialization example there is Fye, American Cardiology
- For orthopaedics itself, there are two to consider.
1. a longer one, UK based: Cooter, Surgery and Society in Peace and War
2. a shorter one, US based: Gundle, Rearticulations of Orthopaedic Surgery
