The biggest issues I see are:
- 
delays due to cutscenes and animations within the dungeons that some people have exploited or found some way to avoid those, likely outside of Blizzards design intent. 
- 
Some classes can shortcut through or over certain obstacles in certain dungs by using class specific skills that most everyone else has to run around. This could be by design or an exploit, I don’t know. But its one rule for those and one rule for everyone else. 
- 
If you don’t start it on day one and keep on top of it, you won’t rank very far at all. Due to RL issues I wasn’t able to start the event until two days later and the leaderboards were already fully stacked. I tried to pile on runs in an effort to catch up but all the horses had well and truely bolted. Killed my motivation to even really try after that. 
It’s a fun event, but implemented poorly.