Why are you using 5/8" material? Also, what are you planning on putting on the shelves? MDF hasn't the strength of solid wood, and will break with little provocation.
This simply isn’t universally true.
We’ve been designing with a German cabinet line that has been using 5/8 particle board for decades. Our clients are the sort that don’t pay top dollar for an inferior product. There are tens of thousands of these kitchens stateside with stone and concrete tops - some hanging from the wall. None to my knowledge have suffered breakage in everyday use. It all depends on the design and material quality. Now , if a forklift runs into them on a loading dock........
One can by junk particle board or junk hardwood just as easily.
Ace, I don’t get your initial hesitation. Even if there was blowout , the shelves conceal the mortise hole anyway.