There's a case study for just about everything out there, but of course there are freak crashes with huge vehicles that are simply not survivable and may not reflect the vast majority of crashes.
The bigger problem is that even the one major study we have on the topic is very limited.
The data it used is now over 10 years old. Vehicles and carseats have improved a lot in that time, and perhaps education too.
That study had some issues with sample sizes. Some findings simply were not statistically significant. Too few infants under 12 months old were in the group of front-facing occupants, and too few toddlers over 12 months old were in the group of rear-facing occupants.
There is an assumption, included in the discussion of this paper, that injury tolerance increases with the age and development of the child. It was always assumed that in frontal crashes, it was the youngest and least developed children most at risk. When the authors presented this paper originally, they were not able to explain why babies under 1 year old had lower risk of injury in frontal crashes, other than the sample size for the comparison group was too small to make a valid conclusion. They were also surprised to learn that the benefits of rear-facing appeared greater in side impacts compared to frontal impacts.
The same problem exists in other countries. For example, in Sweden, so few if any children would be forward facing in a harness that a statistically significant comparison could not be made.
So, I believe it comes down to a few things.
1) Based on the undeniable physics involved in frontal and side impacts, rear-facing is safer for occupants of any age. This assumes that an appropriate vehicle seat or car seat is available.
2) Older, taller occupants are at greater risk for head injury, due to the potential for the head to strike a hard part of the vehicle interior. Head excursion related injuries are generally considered to be the most likely to cause serious or fatal injury, even moreso than internal/spinal injuries.
3) As young children mature, their bodies become more resilient to injuries typical of automobile crashes.
So, I think there some point where #3 becomes a bigger factor than #2 and the benefits of rear-facing begin to diminish, though there are always benefits.
The problem is we simply do not have enough research to say at what age that point is. We also have no data to say how much safer rear-facing is for children above 24 months old. To be honest, the published data we have for children under 24 months old is limited basically to one study with some admitted flaws, so a lot more research is needed! In the last 10 years, we have safer cars and carseats and a lot more kids riding rear-facing beyond 12 months, so it would be interesting to know more.