Solar farms are definitely much, much simpler .. thousands or even millions of perfectly uniform panels manufactured in a factory, some wiring and power electronics and some electrical infrastructure to connect to the grid. Projects are delivered in months, and almost all within 1% of project budget.
Nuclear power plants consist of probably tens of thousands of different components: a reactor (containing a reactor core, fuel rods, control rods, moderator, and coolant), a turbine, a generator, a containment building, a cooling system, pumps, valves, and piping, a control system, a safety system, and a waste disposal system, along with all the same electrical infrastructure to connect to the grid. Projects take years to decades and invariably delivered multiples over budget.