Drilled will crack,m but it's nothing to do with brake dust doing it, it is bcause of the different rates of expansion of different thickness metal, that's why the cracks always start at the thinnest part in the bottom of the hole, same as pads cracking is not a fault.
Mosty of the brake dust is resin used to bond the various mixes together on the pad plate, if it were hard enough to crack discs, do you really think you would simply wash it off when the wheels are dirty with brake dust.
Any surface that is not uniform and heated will develop weak spots, as some areas by defintion are not as strong as others.
As to how much cracking is acceptable is up to debate, I've had discs split on vented/solid drilled grooved etc before now, these are replaced, but cracks around the holes, usually means you would benefit from larger discs.
Porsch put shed loads of holes in their discs as do other performance cars, interestingly Porsche are NOT countersunk.
tabetha