In battles, I tend to find I get much higher returns on cost using lots of cheap units than fewer expensive ones if stack size isn't an issue. For example x2.25 units of normal slingers will beat a Judean slinger easily. x2 militia hoplites will bear a normal hoplite easily.
I know population can be an issue, but if anything it's usually because you can't afford enough of the elite troops. More to micromanage can be a small issue, but again, not in non-maxed out stacks.
What am I missing?