Stelios, the host, is a great guy who is always reachable in case you need something. There are nice coffee shops downstairs and a decent supermarket very close by.
The area is perhaps not super clean but is very close to the Omonia metro station and very walkable to Monastiraki (say 12 min or so) and Syntagma Square (~20 min) as well. I have generally felt safe there, even late nights (slightly inebriated), and have not been bothered by anyone. Cabs to Syntagma Sq costs something like €3-4. Use Taxibeat app for cabs, much easier.
The flat itself is nice and the balcony is very nice when it's warm.
I also used the efood app for food deliveries and it's fantastic (once you figure out what a pie is).
Overall it was a great stay and I'd recommend this flat for anyone needing a centrally located place to stay.