Speed factor.

Although, I've also had situations where the AI, knowing valuable spells, for some reason didn't cast them, and therefore was defeated.

Well, here's one of the reasons why I don't consider the AI a worthy opponent. You attack it, and instead of using an offensive spell, it casts some kind of air defense: D

I hope you don't take what I said as rudeness? :)

Rudeness? Not at all. Everyone enjoys games in their own way. I personally enjoy competing against other players (online or offline - it doesn't matter). For me, the whole point is to do something better than my opponent. I have no interest in winning against the computer. For example, with the First Blood 2.4 map, I was initially interested, but after playing for 2.5 weeks, I realized that I would complete the map in about 2-3 months, and I immediately became bored because I knew the solution. I haven't returned to that map since. If you enjoy completing scenarios, I can only be happy for you.

If you really want to, you can take two armies, for example, Castle and Fortress, with a month's worth of fully built cities, two heroes of equal strength, and pit them against each other. Who will win?

There's a map called Tactician Corner 3 (TC3) where a battle takes place between two races, each with 10 units. You have to choose the 7 best units out of all 14 types of troops. And no one lets you play with the Swamp faction there because gorgons, hydras, and basilisks are seriously powerful.
Also, besides strength, you need to consider the cost of units and buildings, the heroes that are heading to that city, and many other factors.

By the way, you still haven't answered who you prefer to play as.

I don't care, to be honest; I try to play as everyone. But I really like Inferno, Conflux, and Tower.

I thought about the scorpius, but I couldn't bring myself to call them "cats."

Well, considering that they have a lion's head, and a lion is a member of the cat family, it's quite acceptable. In any case, they look more like cats than ancient monsters look like bears.