Honestly I think the biggest issue stems out of Dev theory vs Player reality.
In the Dev theory, the playerbase goes into Eureka, groups form to tackle zone mobs together and level and that will eventually trigger the NMs which groups can come together to kill. In Player reality, however, we figured out that if we can just steamroll certain mobs to spawn NMs, we can get large amounts of EXP and crystals with little individual effort. The Devs for FFXIV seem to have a really hard time considering what the playerbase is going to do in regards to content. This is actually nothing new and this video is a prime example of how long this has been an issue: https://m.youtube.com/watch?v=KFNxJVTJleE
Eureka and Diadem both are absolute case studies on the necessity for a public test realm where a subset of players can interact with and test content of this scale before it is released. The dev team simply does not have the manpower in their ranks to adequately simulate the true actions that the player population will take. A PTR would go a long way to helping make this type of content better all around, and no I don't think that anything that has to do with raid progression or such should be player tested. The devs can handle small instances like that, but anything where you're putting large amounts of players like this into an instance is something that we should be able to give feedback on and it's the one thing I feel is holding back content of this type in XIV.