I recently stumbled upon Yandere simulator. A game in development and with plenty of video blogs to follow the progress.
I have often surprised people have met with how long I have been working on BSS/BZS combined (since 2007/2011) The ups and downs, the reasons, and in many cases had to explain why I keep working on the mod.
Yandere simulator cannot be compared to a mod as it is from scratch, and it is neither worth comparing to BZSmod due to the vast difference in scope.
But i think there are some keypoints in this video that are general and well explained to cover a lot of question to "why it takes so long" to develop a mod/game.