Don't think APs are only about the internet speed. I have a Synology NAS, so streaming and copying files between devices (wirelessly) is also important.

I live in a three story home, a typical European row house (so completely built from concrete, with wooden doors), and have my Unifi Dream Machine in my utility closet where the electrical board and my home server is, and my a Unifi U6 access point at the top of the top most stairs. It is completely sufficient for 30 wireless clients.

What about latency, disconnects, internal traffic, ... different types of construction materials?

Traffic to the internet is not everything. What about self hosting, sharing files between devices, or simply pushing documents to the printer...?

I have a small flat, but it has an L shape, reinforced concrete on many walls, an elevator right next to one of them, and so on. With typical setup here ("router next to tv") I would have no WiFi signal in one room at all. Regardless if it's 2.4/5/6. And it's just "a few meters". So two AP's with low signal strength are infinitely better.

2? I have 12 APs in my house. 5 ghz propagation is terrible through insulated walls, especially those with metal film and 5/8's drywall. Add the green stuff and a closed door and APs multiply.

Concrete is pretty limiting.