The only problem is that when you play on Champion difficulty, it becomes difficult to acquire harpies...
What do you mean? Are you talking about resources?
Personally, I always choose the strongest units (of course, based on my personal perception of this criterion), not the most economical ones. Of course, in this case, there are sometimes resource constraints, but this choice doesn't really pose a problem for me. After all, there's a market (where you can always exchange what you don't need for what you do), and there are artifacts. And I know that all the costs and difficulties will be fully compensated. The outcomes of battles prove this!!! ;)