Exo-Zombies gameplay-wise wasn't really all that bad. I know Sledgehammer tried their best to follow in the footsteps of Treyarch's Zombies, with the story concept, "wonder weapons", and intricate side quests.
Unfortunately, it was pretty lackluster because they just couldn't figure out how to ramp themselves up while distinguishing their mode as its own marvel. The story elevated into DLC 3 but it was too late - nothing about it kept the player wanting to learn more about the outbreak and what entailed afterwards.
The quests were just very odd to do in terms of steps, and the final "rewards" weren't all that wonderful either. I will give them credit though, because their steps were pretty puzzle-like which made completing it feel like...like an actual quest up until near the end?
If anything, the mode is overall hectic. Which I personally enjoyed a lot because it always forced you to stay on your toes. You could still train around like any other Zombies-esque game; however, their AI movement was very irregular and some zombies like the EMP-zombies made is 20x harder to navigate the same way all the time. Because of this, you really had to plan out your exit strategies if shit hit the fan.
Overall, could definitely use more work. But if Sledgehammer chooses to make another Exo-Zombies, they need to do more studying.