Here's my opinion as to what holds this game from being truly great. As you pointed out, the meat of the game is the tactical combat - that's what you'll probably spend 70-80% of the game doing. Unfortunately, the combat lacks sufficient depth to remain interesting throughout the ~40 hour campaign. There just isn't enough variety in unit abilities or character progression to keep things fresh. Your hero's abilities aren't terribly exciting and you'll acquire most of them in the first two chapters anyway. Once you've leveled up your troops and settled on a successful combat strategy, individual encounters become more of a chore than anything, since the enemy really can't offer much of a challenge at that point. The addition of a fast combat button helps, but is somewhat inefficient and if I use it then feel like I'm skipping the biggest part of the game.
If we compare Legends of Eisenwald to some other tactical strategy/RPGs - like Blackguards and Disciples, for example - you can see that those games offer a greater variety of tactical challenges. For example, Blackguards has large battlefields with traps and other obstacles, as well as a wide variety of character abilities and spells. Disciples doesn't allow for much tactical flexibility, but at least offers a diverse range of opponents in the four different races, each with their own units and upgrade paths. On the other hand, Eisenwald has a single "race" - human, and you'll quickly figure out the "best" unit type and how to counter enemy units of the same type. As I mentioned above, hero progression doesn't bring any notable diversity to the combat either. At least with Disciples, you had an extra city management/global spellcasting mechanic that spiced things up a bit.
I actually feel that Eisenwald's story is solid, but it is the setting that somewhat limits the available combat diversity. Of course, it's unreasonable to expect mages to be casting fireballs in medieval Germany, or to fight with actual demons. If you guys do decide to iterate on the formula, I think a more traditional high fantasy setting, like the one in the recent King's Bounty series, might actually be more fun/interesting. Alternatively, if you decide to stick with a historical setting, you can at least try to make the battlefields larger and the combat situations more varied. For example, you could have ambushes where you're attacked from two sides, or three-way battles between you and two other hostile forces. Additionally, sieges can be improved by having a narrow path with a gate/drawbridge that your melee troops would have to break down before they could engage the archers on the walls. Elevation would help improve ranged combat, etc.
Again, I feel that, despite its flaws, Eisenwald is a really solid first effort from an unknown developer, and I would encourage you to keep making games that you like, whether or not they become bestsellers. If you manage to build a loyal following through a series of solid games, then commercial success will follow. I know I'll certainly be inclined to support your next game, if you decide to return to Kickstarter or fund it yourselves.
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