I don't know if you saw
this article that I linked some time ago in another thread.
It is my impression that if the manufacturers were going for maximum reliability they would still be using ceramic substrates.
Ceramic - Silicon ~ 3.7 ppm/C difference in CTE
FR4 - Silicon ~ 19 ppm/C difference in CTE
Of course with ceramic, there is more stress on copper/ceramic interfaces, and shearing of vias within the substrate may be an issue? Lots of tradeoffs here.