But those problems exist for large coal/etc plants as well, so some of that is baked into the equation and doesn't do much to explain why nuke plants end up paying 10x as much for a plant that is only slightly more complex.
My take: one significant issue is that nuclear power is the only large scale electricity source with meaningful, consistently enforced regulations.
There's no equivalent of the NRC setting welding standards for the steam systems of coal plants and demanding that control systems are guarded against wiring fires. General safety and environmental regulators are often deliberately underfunded by business-friendly legislatures, and even violations that they catch are rarely considered severe enough to shut down a site until remediation. And of course nuclear reactors are (rightfully!) expected to contain their waste products, while combustion plants still get a free pass to dump CO2 into the atmosphere.
The result is that very few industrial sectors in the US have experience receiving meticulous government oversight that they cannot ignore. Aerospace [1], pharmaceutical manufacturing, and nuclear power are about the only ones I can think of. If contractors don't have institutional knowledge of actually working on nuclear projects (which are already more complex than a big coal or gas project), they may not be prepared for regulators that actually check the work and demand it adhere to documented standards. Just about every other project can let marginal welds or inconsistent paperwork slide with "eh, you tried." The really expensive part about nuclear projects isn't doing the work. It's redoing the work that you failed to do up to standards the first time.
But you absolutely do not want to make the NRC more lax to bring it in line with the barely-there standards the rest of industry is used to ignoring. You don't want the nuclear equivalent of the West Fertilizer Company -- inspected twice in 28 years, minor violations ignored until the plant exploded [2].
[1] You might even put an asterisk on "aerospace" considering the 737 MAX debacle.
