"However, the BBC understands that the fact Sizewell C is a carbon copy of Hinkley - which has seen work on a second reactor there completed 30% more quickly than the first - is thought to have substantially mitigated that risk."
It sounds like it should be pretty easy to budget for this plant. Plans are allowed to include a 20% "we don't know what this is going to be spent on but something will come up" for surprises.
And yet, renewable projects typically come in within 10% of the contracted price.
The modular nature of renewables also allows them to be much more resilient in the face of component failure, or in face of construction errors (with the exception of common mode design failures). This means they can be installed with less skilled labor that makes more mistakes.
Renewable fields are akin to server farms w. large numbers of replaceable blades. Nuclear plants are like mainframes.
Every manufactured object has an insanely long and convoluted dependency chain, where each link is subject to disappearance due to factors completely out of your control. Especially objects that require special or unusual parts.
Then there's actually building the thing, which can go wrong for a million reasons before you've even reached the grand and rolling lands of human error.