The combat setups were really bad. Like the enemy types, enemy placement, flags for when and where new enemies would arrive from. It all reeks of not being playtested enough and instead it's a frustrating mix of deaths out of nowhere and combat engagements that are not fun.
That combined with the story skipping around, not explaining things, and having kind of an abrupt and "done before" climax, made for a game with a lot of problems.
What I'll give U3 is that it has darn good character development. The main cast are some of the most interesting characters in gaming recently. They're developed and portrayed in a realistic way that makes it almost like watching a solid film. A lot of the set pieces are also real impressive and obviously great visuals.
It's worth playing, but it's totally a "Could have been" game. What it does right is the setup for a great game, but the combat makes it unfun to play and the story is such a mess by the end it's hard to care about the narrative when it's over.