Harmony - Troop Symmetry

Yes, the Tower also has flying dragons, but that makes sense – flying dragons are a classic, and walking ones are an exception. Exceptions shouldn't become the rule by being repeated for two factions at once.

Besides, there's a fundamental difference between light and dark dragons – it's about initiative and power. The former have initiative, while the latter are more powerful.

And, well, if a level 5 unicorn is replaced by a phoenix, then of course, replacing a flying dragon with a ground unit is a lesser evil. :)

Why not keep the classic unicorns and replace the dragon with a phoenix? Or was the phoenix specifically designed as a flying unit to replace the unicorn? If so, it would have been better to try pegasi, in my opinion.

And "earth" doesn't really fit with a rushing faction; you'd have to completely rework the entire faction. In terms of gameplay, I'd say that the elves worship a dragon of air, not earth. And it's in this context that the dragons from Heroes 7 don't really fit here. But if you really want to see them, you could consider them as a replacement for the ents.

Yes, I agree – the elves in Heroes 5 are quick and fragile, just like the Emerald Dragon in the original, but it's still earth, not air. The developers wanted to create something new based on the old (they wanted to satisfy the fans of Heroes 3), which is why the elves turned out to be so contradictory. But still, an earth dragon is an earth dragon, and the classic doesn't fit with the new world. I'll add that the concept of "classic" is not absolute. And in the end, why have two identical games that only differ in graphics? (This is, of course, a very rough statement, but still...)
We have Ashan, and the elves worship a dragon god of earth, whose avatars, like their mother, are FAT, CLUMSY MACHINES. So, it would be better to rethink the entire faction based on your logic, but I don't understand why everything has to be black or white (either all quick or all bulky). In my opinion, a combination is more interesting, and besides, there's Inferno if you really don't want to lose a highly mobile army (only the Lords are slow there, but that's necessary, especially considering that they will be given "Terrible Neighbors" – let's rename that)