Unseen Forces is an expansion to Elder sign. I assume that whoever reads the review is familiar with the main game as it is required for Unseen Forces. So it adds 40 adventure cards, 2 Other Words, 8 investigators, 4 Ancient Ones, 12 common items, 12 unique items, 12 spells, 3 allies, 20 Mythos cards, and 9 optional Master Mythos cards (for a greater challenge). It also adds some monsters, doom, stamina, and health tokens.
The main addition however is two dice: blessed and cursed; they are given to investigators at some points of the game. While blessed die words exactly like an extra green one a cursed die exists to make your life more difficult. The museum entrance is replaced by 4 different entrances. You cannot but Elder Signs anywhere; thus the easiest and safest way of obtaining them is no more. To compensate, the new adventures and Other Words offer more Elder Signs as a reward.
So the main question is, does the expansion make the game more difficult? The jury is still out there, even among experienced players. My personal opinion: yes it does, but not bu much. On the positive side it sure adds more variety. Also blessed/cursed dice used in all other further expansions if you want to get any of them. This is not a must for people that did not care about the main game much, but for those that like it I would recommend to get it at least for variety purposes.