There are more than enough on the road already to know exactly how safe they are. FSD has already driven a billion miles. Once it is good enough they will have the statistics to prove it.
Tesla has, as far as I can tell looking through the reports, done not a single mile or kilometer under the Californian DMV Autonomous Vehicle Tester (AVT) Program[0]. Nor have they partaken in any other program of this kind that would enforce public reporting or any measure of transparency. There is no public data to gauge safety or reliability as of yet.
You are right, there is no public data, they've kept it all private. FSD is not anywhere near good enough to be unsupervised right now so there's no real point to doing autonomous testing.
I'm simply saying that when it improves to the point where it can be unsupervised, they will definitely have the data to prove it.
It is far from good enough today, no doubt. But the rate of improvement is what matters, and from personal experience I believe it is quite high. I do wish Tesla would release some better statistics.