If you think about it, what is the argument for why nuclear is to plan? Nuclear is impossible to plan because ... pixies steal the computers the planners are using? Note that whatever factors you identify, the people estimating the plant build process are aware of that.

The planners know exactly how long it takes to build a nuclear plant, and they learn just as quickly as everyone watching from the sidelines. Their assumptions are being thrown by something - almost certainly anti-nuclear campaigners in government or regulatory forces warping incentives. Those are the only things that can consistently diver overruns. Otherwise it is hard for overruns of a nuclear plant to be different from any other project.