In my experience with construction projects, legal costs and liabilities take the cake. Once something is in court, there is zero telling how long and how much money it takes to resolve.
This is interesting from a foreign perspective. I still follow urban planning in Helsinki, Finland. Major public projects always face opposition and go to court, but it's not a big deal.
Because zoning plans and similar project plans are decisions made by public officials, all complaints about them go to the administrative court system. Administrative courts don't care about the substance of the argument. (That's for elected representatives.) They only determine if the officials followed all appropriate regulations and decisiomaking processes. Going to administrative court is cheap enough and fast enough that the costs and delays are usually included in the project plan.
Sometime the court overturns the decision, delaying or canceling the project. Sometimes their justifications are stupid and sometimes there are unintended consequences. Regardless, the system more or less works most of the time.
That seems like a political culture issue rather than something inherent to governments. If you really care about doing good in the world (isn't that the whole point of politics?) then you have a huge incentive to save on costs as any money saved can be put to good use elsewhere.