This apartment unit needs air conditioning to cope with Lisbon's sweltering summer heat. The air does cool down at night, but then I am faced with a no-win situation - I can open the windows and endure the loud music from the pub downstairs, or I can close the windows and sweat. Neither choice is appealing. (Don't get me wrong... I like Destiny's Child as much as the next person, but there's only so much I could handle at 2 AM in the morning.) As a data point, when I open the window to the back, I can see that most neighbors have air conditioning.
The second issue is the uneven floor boards. Some of them sink down when I step on them. As a result, the back of the nail sticks out. It won't puncture skin, but it does make walking around barefoot quite an exciting experience.
Overall if you could avoid visiting Lisbon during summer and plan to wear shoes inside the unit all the time, then it's probably OK. As for me, the hot temperature made our stay quite a miserable experience. (And that was not even during the heatwave... I can't imagine what it's like during the heatwave. )