I've got no idea what people are talking about, when they say the game is too hard. It seems just hard enough. I didn't have to pay anything to win on the highest difficulty - although it did take intelligence, luck, and a few tries. It could use a bit more polish, but it seems like a very balanced system. Indeed, I was nearly tempted to purchase the add-on.
After reminding myself that I won't pay to critique any game on NG, I had to sit back and wonder what might prompt that last star, in my eyes...
The odds of an enemy attacking needs to be adjusted. I usually coast through a game, sitting at about 33%, and I'm certainly not attacked 1:3 times. In fact, I once had the probability all the way to 100%, and I didn't get attacked.
I also think that more technologies should be added, the requirements dropped somewhat, and the bonuses they give a little more nerfed. This would allow players to specialize their tribe a little more.
Perhaps the option to attack, rather than just sit and wait to ~be attacked~ would be nice - temporarily lowering the exterior threat to your tribe for doing so, and maybe even discovering a new tech or gaining some resources as an extra.
I like that you've got 100 rounds to finish. Keep it.
I don't like that you've got to wait for a certain time to unlock other techs, and I think that it should be more based on the level of evolution you've achieved.
Finally: I'm a critic, so I'm not writing any of this to be a jerk. I love the game just like it is. My thoughts and a 4-star rating exist as an encouragement. If you can get me all the way to 5 stars, it means that I like something enough to pay for it. Upgrade this baby, and we'll talk price.