There are multiple reason for why ZTD was unsatisfactory but I think it still has some value. The problem is that you're stuck in a point in time between the two other games and your goal is to make sure none of the interesting stuff in the future even happens.
The fragment mechanic was cool because it perfectly mirrored both the memory loss as well as the feeling of jumping between timelines.
Unfortunately, much of the story was already set in stone in a way where the questions are always gonna be much more interesting than the answers.
It's nice that it addressed the futility of time travel in a split timelines universe, which is a question VLR dodged entirely. Trying to change the past by preventing the outbreak was an entirely selfish endeavor by Sigma and the others, which is cemented by the "good ending" decision leaving the group's alternate versions to die for something they had no part in. The ayy lmao technology was kinda awkward but it was an important inclusion to make sure that the events of ZTD had to happen in some form. It's essentially just a way around the "But what if Brother wasn't born/didn't get sent back in this timeline, how is he still here?" question, which is typical for time travel stories.
What they really fucked up were the characters. Junpei is too edgy and shouldn't have been part of the experiment. Akane is not nearly assertive or cold enough, by this point she has been "Zero" for ten years and according to the director, Akane is supposed to have the strongest ESP/SHIFT powers. Sigma is supposed to be an old man but only occasionally even comes close to acting that way. Sigma and Phi also more or less mastered "shifting" in VLR, yet they don't utilise it at all in ZTD and instead Junpei and Carlos lead the charge on the timeline adventure. Diana was a delight. The entirety of Q-Team just feels completely out of place, Eric in particular is really frustrating to watch. They completely neglected to show what happened to Santa, Clover or Alice.
Brother causing the outbreak to stop nuclear holocaust could be an acceptable twist but it feels very weird.
The implication is that he goes on to create the Myrmidons for the sole purpose of creating Dio so he can infiltrate the game in VLR to place bombs to encourage Sigma and Phi to train their ESP to go back in time to ZTD and make sure Brother is born.
The events of 999 also only happened because of Brother, meaning the entirety of Zero Escape revolves around Brother trying to make sure he is being born.
The highlight of the game is definitely hearing Diana speak those words we know from VLR into the radio, revealing their meaning. In my eyes the pandemic ending is the "canon" ending that feels the best