This is a very complicated subject. Back in 2008, Bill Finley, who writes about synthetic surface handicapping (among other horse racing topics) for ESPN etc., reported that the % of breakdowns between dirt and synthetic were not wildly different. As I recall, a fraction of a percentage point or more. Don’t hold me to that, I once sat on Sun Beau’s tombstone so long ago I can’t remember where it is. I do know that oddly most of the research and publications on synthetic surfaces are British. I’ve read a number of them. They are very informative, but none of them, because of the technology is so new, have any idea which poly/all weather surface produces better safety. Whether a dirt, turf, or synthetic surface succeeds as safer is based largely on the sub-surface construction and drainage. Synthetic surfaces are extremely high maintenance and expensive to maintain. It is no surprise to me that tracks are returning to dirt surfaces for that reason and not only because most races in the USA are on dirt.
I recall that Dr. Rick Arthur, CA state veterinarian, has been often quoted as supporting the “safer than dirt” theory for synthetic surfaces. I am not aware that he has changed his position recently.
Recently, the Paulick Report had an article on breakdowns at Charles Town. I think it reported 147 deaths over the past 12 months. Its bull ring track is dirt and more dirt. Now if you question the safety of dirt tracks check that out. It has been a number of years since I have traveled there, but I suspect no one has a clue what is geologically or man-made supporting that track surface. JMHO, but in the case of the minor league and historically older tracks, it is reasonable to suspect that very little attention is given by the track owners to sub-surface maintenance and renewal. Collapsing or failing infrastructures such as road bed and drainage gradients and fixtures go completely unnoticed, thus contributing to unstable surface performance. Many should recall the drainage problems at Santa Anita when it switched back to dirt. Flooding and more flooding as I recall. Stronach just removed the synthetic material and layered on the dirt one. No sub-surface renewal at all.
I offered up the Bill Shanklin link as informative, so I apologize if some of it might have been contradictory. Consistent is in the eye of the beholder. Three horses breaking down on Preakness Day might be consistent for Pimilico, if you think horse racing is lacking in caring for its horses or you are an uninformed sports writer who normally covers tennis, but inconsistent to the trainer or keen turf writer (who are almost extinct) that shows up for the morning gallops on a daily basis.
As for Del Mar, it is a flip of the coin or the position of the moon as to what the benefit will be. Some days at any track are worse than others. Maybe the racing secretary should carry his prayer book and rosary beads.