I know 120 athletes who have used and some have been injured but as of yet no fractures.
Stress fractures are usually due to too high a volume coupled with too little recover and poor monitoring - completely the opposite of what a short to long philosophy promotes. Someone who is getting stress fractures from running 2 high intensity sessions of 500m a week is doing something seriously wrong or ignoring or not monitoring the status of their athletes (especially their feet - where you should stop immediately if they say they have a problem). Remember the whole point of coaching is to come up with your own sessions that fit what your athletes can do given your own circumstances not just copy some program you think is “short to long” that you saw on the internet.
Other less thought of reasons for stress fractures could be too little Vit D and also foot mechanic issues such as too high a rate of pronation at the end of range.
Think things through critically and then make up your own mind about what training programme would be most appropriate for your athletes. What are the benefits of short to long vs what are the issues? Could you implement this in your current situation? Would your athletes benefit from this kind of focus?