The new OL3 reactor at Olkiluoto power plant is an EPR reactor with nameplate capacity of 1600 MW, i.e. it was the first of it's kind anywhere. With all the delays in Olkiluoto project, there are now two units with similar design running in China so it will not be the first one to start.
Reason for the delays mostly has to do with the project model, i.e. having two vendors from France (Areva) and Germany (Siemens) working for the Finnish client TVO. Poor management by the client has allowed the vendors to point fingers at each other on various issues, so not only is the plant completing a decade late but there's also a court battle brewing between the parties. Most of this has nothing to do with nuclear power per se, however.
In a sense it is a problem distinct to nuclear, at least in the energy space. Reactors need to be so big and are so complex that they're vulnerable to horrific project management failure modes. Avoiding those failure modes takes experience at a level very few people get to operate, so skills are rarer than projects.
The fact that nuclear projects require this sort of huge, long term projects, instead of a quick buildout as compared to solar, does indeed mean that all of that has to do with nuclear power.
When you're not building on a new design at a scale never done before, it is a very different story from the case of OL3. Just take a look at how the French built their currently serving reactors in the 1970s - more than 30 units with the same design were build in a timeframe of about 10 years. High degree of standardization has significant benefits also in the nuclear space.
But of course you're right - it's very easy to deploy a bunch of solar panels compared to building a nuclear plant. And once you're done, you get to start to figure out the energy storage part - which in many practical settings is still mostly an unsolved problem.
Reason for the delays mostly has to do with the project model, i.e. having two vendors from France (Areva) and Germany (Siemens) working for the Finnish client TVO. Poor management by the client has allowed the vendors to point fingers at each other on various issues, so not only is the plant completing a decade late but there's also a court battle brewing between the parties. Most of this has nothing to do with nuclear power per se, however.