All the 1 star reviews I read mention the same issue: bosses are hard. Not near-impossible like these reviews claim, but you do need to ask yourself, after a few attempts, "what do I need to do differently to beat it?".
Indeed, during boss fights, you have to learn to dodge and hit, and I can see how some feel discouraged.
That being said, I wouldn't give this game a perfect score, as I think several aspects could be more polished with a difficulty that is more even between the mobs and the bosses, but it is a solid action and customization game, albeit a bit pricey.