- What are the drawbacks of Heroes 3? (What didn't you like about it?)

Write it subjectively too... I didn't like the graphics....

No, sir, objectively. From the point of view of the technologies used, the graphics in the fourth game are BETTER. I wrote about the design - "subjectively."

of course, it's more interesting... in Chaos, there's the Haste spell, and in Nature, there's Speed, etc. It's definitely "much" more interesting... it's just a complete copy...

But here, please, elaborate - about "etc." and "complete copy."
The magic in the third game isn't much different from the magic in the second. We build a guild, increase Wisdom, get spells, determined randomly/probabilistically. Very interesting.
It's a different story - full-fledged schools of magic, implemented in the fourth game, without any "complete copies" (spells are repeated, but each school clearly differs - in the direction of most spells, each school has its own specifics).

and again, IMHO... yes, there's the possibility of mixing classes, but hero specializations have been removed, i.e., it's again just a copy of the same thing, not individuality...

That's exactly it: in one case, we are offered a predetermined "individuality," and in the other, a very wide range of possible hero development paths with priorities set by the player. Which is more valuable and interesting?

"The Fork" is an obvious and unsuccessful plagiarism (subjectively)... the absence of upgrades... the reduction in the number of monsters in one castle, and considering the difficulty of building higher-level structures, you spend half the game running around with only three different units...

You didn't explain in what way it's "unsuccessful." The absence of upgrades is not a big problem.
No one forbade capturing other cities and developing faster - it's not "half the game."