How accurate is the math of monsters built using the Simple Monster Creation ruleset in Unchained when it comes to the math expectations for monsters?
I'm not sure what you mean by "accurate". Creatures created by Simple Monster Creation have stats that are roughly appropriate for their role and CR. If you compare them to whats in the Bestiaries, the Simple Monster Creation stats are in line with the averages for each creature role. So it satisfies the "math expectations".
I've used them myself to benchmark custom NPCs and monsters, and I've always had pretty solid results.
That is what I meant by accurate, yes.