Olkiluoto 3 suffered significant delays and cost overruns. My impressions, based on recollection I got reading the newspapers during these two decades, are:
1. Bad management of construction project, almost near corruption level stupidity. Many constructions tasks outsourced to contractors who outsourced them to contractors who outsourced them to contractors, making it easy to take "cost-saving" shortcuts and difficult to supervise.
2. Failure of corruption to speed things up: Finnish nuclear safety authorities are not easily bribed, so when issues with bad quality of welds, concrete, boiler valves and safety electronics were identified (all this from memory, these were separate incidents), the issues could not be smoothed over and ignored with well-placed thick envelopes. The authorities would demand the construction crew to tear stuff apart and do again, properly. And if it didn't pass, then again. And again.
Most of the issues I have read about do not sound like "nuclear technology" failures, more about failures in building a big pressurized boiler power plant. The nuclear part comes in by authorities requiring high quality. They couldn't make good enough concrete bed on a first try, that isn't some super novel space technology.
It mostly reminds me of similar kind of mismanagement that plagues many big construction projects run by big corps managed by suits. During same time frame city of Helsinki wasted amazing amount of money in a "automated metro car" subway upgrade project they bought from Siemens, which ultimately didn't work and had to be scrapped.
(edit. City of Helsinki decided to "automate" city subway metro system in 2006, contract signed with Siemens in 2008, originally planned to be completed in 2011. Project was postponed several times and effectively scrapped in 2016. Tried googling most recent news, the lower district court had given judgement in 2021, parties likely to appeal.)