I've built several large-scale buildings and developments at urban scale in California and Nevada, both heavy heavy union states.
I can't point to the systemic problem leading to the delays but I can tell you that any job even tangentially related to construction requires high wage labor. This is universally true for civic projects.
Some of those unions (e.g. electrical workers, carpenters) provide exceptional training services for apprentices. Others (e.g. the people holding the stop signs at highway construction sites) are mostly strong-arm groups.
These groups could be causing delays and from personal experience I can tell you they do, sometimes. But overall I can certainly say that the point you made here is not the reason why projects are delayed.