People are right on both ends of the spectrum here. Many people get implants and many people can keep them for decades without problems. Once put in, you can never go back to good as new. Here's a list of problems that can and do happen:
- Implant erodes and the fragments cause foreign body reactions.
- Bone erodes under the implant. I'm skeptical about this one. Bones shrink with age anyway.
- Improper implant contours. What's flush with bone at 30 probably isn't at 50. Good facial balance at 30 becomes bulky at 50. Implants don't recede with the rest of your face.
- Implants can migrate. Bones change, hardware moves, things loosen, etc.
- Implants harbor infections. Years or decades later. All surgeons see these, even those frequently mentioned here.
- Not all are removable. The ones with "bony ingrowth", not coming out. Like the new CT-bone ones.
- They can compress nerves. Particularly those infraorbital rim ones have to run around infraorbital nerve.
- Damage to soft tissue around them in surgery can be unaesthetic.
- They are hard to get symmetric and can become asymmetric with time
- You might not be a good surgical candidate for removal later if they cause problems
- 50 years is a REALLY long time for anything to survive, in or outside the body. It's 2016 so take a look at objects made in the 70s for comparison.
- You can only augment to today's deficiencies. As your face recedes you might want more to remain young looking.
- Osteotomies have their own problems. Sinus cavity exposure from jaw surgery is bad enough for example. Unpredictability. Difficult to access hardware. Irregular contours. You didn't ask, just to name a few.
On the plus sides:
- Bony deficiencies create other problems. Eyelid problems, dry eyes, etc
- Some people are unwilling to accept normal airway jaw surgery due to aesthetic compromise. Implants make it possible.
- The risks of implants may be better than facelifts
- Mental health problems from lack of confidence are no fun
The point is, yes implants really do have problems but just because they have problems doesn't mean you shouldn't do them. The alternatives have problems too. We don't get the option of a problem-free life. My impression is that doctors think that 50s is the optimal time to consider them because you're unlikely to live long enough to have to deal with the problems for long if you do. And, of course, the longer you wait the better your options will be. Who knows, maybe by then non-surgical solutions will exist, like growth hormone injections. Surgical options will improve, like lab grown augmentations, accurate soft tissue models, resorbable load bearing bone cements (+ benefits from less retraction), etc. Once you get surgery, you're stuck with those compromises for life.