Walkthroughs for custom maps.

Decided to return to Dark Man..

This is both the strength and the weakness of such maps. By making the world open, unforeseen events can always occur: the AI flying from one island to another without having the necessary magic, the AI getting stuck near a creature dwelling and refusing to open a quest location, the desired enemy hero going in the opposite direction, and as a result, a tent not being opened. All these and some other problems have already occurred in Heroes 6424 and on other maps. I think that on such a dynamic map, the author is unlikely to calculate all the options.
Therefore, such shortcomings are a kind of "attribute" of such maps, and I personally am willing to put up with them.

Next, regarding the Grail, on some maps the AI can indeed dig it up. Usually, this is not critical, unless the author wanted to force the player to place it in Flax to learn magic. Let's just say that on this map, losing the Grail is tolerable.

Regarding the exchange of troops between heroes, I have already shared my opinion on this: usually, it simplifies the game. Instead of several, usually ALL MORE POWERFUL than us, heroes, we get several weak ones and only one strong one. By killing all the weaklings, we gain control of the map, and we will kill the strong one when we gather our armies. In my opinion, this is a common practice. Yes, if we are playing according to the author's plan, then we kill enemies as soon as they appear (this is usually the plan), and the players do not have time to exchange armies. But in my experience, such an exchange is not critical. If it turns out that all the super-powerful units are gathered on a hero with a Sphere of Inhibition or a Cloak of Negation, then we will simply leave this hero for last, nothing terrible (unless, of course, it is a quest hero). To prevent this from happening, experienced map makers usually do not put inhibiting artifacts on the strongest enemies. And any army on any hero can be killed over time.
So, with the statement

After all, theoretically, some hero can acquire such an army and such artifacts that it will become simply impossible to defeat him.

I cannot agree. Any enemy can be killed, the question is only of time, strategy, and strength, as well as the possibility of hit-and-run tactics.

But I agree that it is possible to leave enemies without a castle, which will lead to the loss of the hero and the map becoming unpassable. In any case, the author needs to think about such a scenario; this is a significant drawback.

But I repeat, getting such a UNIQUE map as Dark Man, on which everything would be "in order," is probably impossible. Therefore, I personally put up with its shortcomings as an integral attribute.