Thoughts on the scenario after completing "Hammers of Fate".

Well, it might have seemed that way because you got the hang of the first two, making the third easier to complete. The fifth mission for the Orcs is certainly challenging, but it didn't seem too difficult to me. The key was to immediately start pressuring the orange player and not get distracted by the turquoise one. The first mission for Zephyr was much more difficult (and still is).
In other words, I want to say that different players get stuck at different points, and the same add-ons/missions can seem very difficult to one player and extremely easy to another. This probably depends on the tactics they use, which race they are most used to playing, which map size they are used to, and so on.
As for the developers making games primarily for us, it's impossible to please everyone anyway. If they had created 8 full Campaigns for each race in the add-on, they would have received complaints from Orc fans, arguing that in total, Heroes 5 has at least 2 Campaigns for each race, but only one for the Orcs. And if they had made 9 Campaigns, with 2 for the Orcs, the public would have complained that there were 2 Campaigns for one race in a single add-on... and so on.
Therefore, they did what they thought was best, and in my opinion, they were right.

I completed the fifth mission like this: I immediately captured the second Orcish town, then jumped to the Knight's Castle. There are strong neutral creatures along the way; I used Astral creatures to bypass them and captured the town. I returned to my town, bought as many creatures as I could, and then attacked the turquoise player, who was weak and easily defeated. Then I went back to my town. This took about a month, give or take. After that, I attacked the orange player. I also completed this mission without any difficulties. The main thing is not to stop and do everything quickly, and I also found the Tear of Ascha. The last mage cast the spell of racial unity, and the centaurs and cyclops stood up, and I took the bloodthirsty ones instead of the free ones, so I had to reload. The first mission for Zephyr seemed a bit dull to me. Zephyr learns a new spell, so he should become smarter, but instead, he becomes dumber by ten levels. And how is it possible that at level 25, he has only 5 spells in his spellbook, and that's only if you take the Phantom bonus. This is not a challenge; it's just stupid. I want to try to complete it without summoning the Castle. And regarding getting the hang of it, I played the mission where you defeat Nicholas five times, and each time I wasn't sure if I would succeed. Or the last mission with Isabel, in the Lords of the Realm campaign; I was confident that I would succeed, even on the heroes. And you are absolutely right that it is impossible to please everyone, and besides, everyone wants something different.