Some context here: https://en.wikipedia.org/wiki/Thessaloniki_Metro
This seems to have flown under the radar a bit but it's a similar case to the metro project in Ho Chi Minh. It took an eternity to finish with the first hole being dug in 1989 and it finally opened to the public last month. It had become kind of a meme due to all the delays and changing of hands among now-bankrupt companies and with every government promising to open it (it's had at least two opening ceremonies years apart). Now a month later it's still having issues and is really small, mostly serving the center but it's the first driverless metro in Greece with modern stations and also featuring a museum so it's promising.
Yes, Athens is saved by its metro. It's like a whole parallel world of orderliness and tranquility under the chaotic car-infested streets above. I would carefully study the city map and basically avoid going to whole neighborhoods unless I could get within striking distance by metro.
The Athens soil is obviously an archeological treasure trove, I believe that explains many of the construction delays. It's the same story in Rome.
The impressive incubation period also explains why it doesn't even have platform screen doors. But otherwise it seemed to me quite spacious and well-built.