Ayoo, hello there! Allow me to throw some shade:)

Yeah, this metro has really been a huuuuge pain over the years and really has various problems. Coupled with the fact that 2years ago we had a fatal crash of trains (one of which was probably carrying illegal oil) and the government is still trying to suppress the case (and there have been almost no changes/fixes to our railway), I've become very hesitant with this metro.

Issues with it:

• A tram (as experts say too) would be easier/faster/cheaper/better to build (also it takes around 3minutes to go down to the platform, a tram wouldnt have such issues).
• It cost around 3.5billion euros, about triple the expected cost.
• The tests haven't finshed yet. They rushed it (our country has issues with railway safety..). The tests should last 18months, but at the time it was opened to the public, only 13months had passed. During the non-public test period, the carts once crashed head-on at minimum speed. During the public test period, a cart once lost power and all of the carts had to stop to prevent a crash (people had to walk inside the tunnels to get out), the statiom has paused operations for other various reasons (once only one route operated for a few hours I think), during opening days there was a big storm and there was water leaking in the platforms (they rushed it that much that not all of the tents were build outside the station entrances).
• It was that rushed, that you couldnt pay with a bank card for some time or they didnt have change for bills over 10 euros, etc. It's a mess.
• It's short as you say, it practically mainly helps commutes towards the center (in the west side or the far east there is no good public transport and it stops after midnight). It doesn't even reach the airport or the interurban bus stations. Only the railway station, which I'd advise not to use.
• There were ancient findings in those areasa they dug. Lots of them. Archaeologists weren't allowed to treat them they way they wanted. (Could be easily avoided with a tram or by letting archaeologists do their work properly.)
• It essentially to 40 years to make it. The main part of the construction took place the last 20 years. The city had increased traffic issues (and looked kinda bad) for 2 decades due to this (now there is an unecessary huge road being build near the ringroad, which causes a lot of traffic in the already dangerous ringroad).
• While it's driverless, it doesn't operate all hours, it stops at 00:30 on Mon-Thu and at 02:30 on Fri-Sun.
• While it's curently cheap and during Friday 18:30 till Sunday 02:30 is free for the public, I think they should make public transport free for the public (what a crazy take, right?), especially for a city that only had buses for the public all this time (decades ago there was also a tram for cargo from the port, but it's been removed). They should follow Belgrade's example and make them public.
• They paid 30thousand euros for the logo (which is essentially a lowercase "μ"), to a somewhat ghost company. Just goes to show how corrupted this whole thing is.

Corruption has ruined it and keeps causing issues:/

The main cool thing I found is that at the platform, there are routes for both directions, so you don't have the angst of picking the correct entrance on the surface when you go to the station.

PS. I dont know if you're Greek too, but if you are, we have mildly active small community at https://fedia.io/m/Greece (is !Greece@fedia.io the corect link?) :)