The difficulty is ever-evolving. I felt like the version Retomation played was too easy. At the very least, it was easy until 1-2, which is when you'd go on to win or get stopped dead and lose all 3 lives. When we added abilities, shields, and potions, that just made it even easier, so we had to ramp the difficulty significantly. It is probably too hard right now. I can beat it with some extremely cheesy tactics (because I know all the tricks), but without doing that I do not win very often.
New people will never know what the older versions were like, so we're always trying to keep the posted version difficult enough. If it's too easy, someone will play it once, say, "That was neat" and never look at it again. The long term goal is to have progression unlocks (like most roguelikes) so that even a failed run will be worth it.
What I love about your feedback is the specific examples. Anyone can say, "You should put in synergies," but giving us cool ideas for them is what is really helpful. Synergies, clan bonuses, and unit combining/upgrades are all on the list of potential features. Eventually you have to sit down and come up with them though and you run out of creative ideas. Those are all really creative ideas that are going in the idea bucket.
Here's a little secret (don't tell anyone): the minions are already divided into 4 distinct clans in the game code and have thematic stats and abilities: The Fiendish, The Enchanted, The Corrupted, and The Undead.