It was a really odd choice considering the mirror universe is one of Star Trek’s classic choices for filler episodes next to time travel and holo deck shenanigans.
I wouldn’t say anything in particular sucked, it’s just that there’s nothing that sticks out. It felt more like a low budget film, but it’s not as thought provoking as low budget sci-fi films. I have to agree with the other comment, it’s very forgettable.