“Go stop go syndrome” is a big one:
The repeated urgency followed by stopping the project that is later urgently started again has a psychological effect on the development team. The developers come to no longer believe that there is any urgency. In fact, they start to get the mindset that this project isn’t a serious project and that it will very shortly be stopped again, so why put in any effort at all?
If a project is important, commit the resources to get it done.