Unit cost and quality.

I enjoy the visual appearance of the nightmare creatures. I rarely build Ifrits because I don't like them. But regarding the topic – has anyone considered not just the construction cost, but also the cost of the building required for that construction? That's why I rarely have griffons in my army. Because they require a Stronghold (which, considering that it no longer increases growth, is better to build somewhere towards the end), while unicorns require a Grove, which costs 5 times less and provides noticeable bonuses. Of course, unicorns are clearly weaker than griffons (but blinding is a useful ability). The same can be found in other cities. However, this may not be noticeable in a long game.