When setting up the devices, the setup on both devices asks to setup a SSID. It even insists you connect to it wirelessly to continue the setup.

But I just want one device to connect to the other, to extend my wired ethernet to another building. I don't want any clients to connect to it. In that case, do I need to configure SSIDs. I can skip the SSID step at setup. However that doesn't mean that no SSIDs are configured, a SSID based on the device's mac address is created,  even worse these are open networks without authentication or encryption.

You can however delete them later on. But I do get frequent and long disconnects at my POC setup. Is this because the devices need the SSID's to connect to each other. Or is it just because the quality of the devices is substandard?

I would like to try different settings (channels, mode AC only instead of A/N/AC auto), but everyting I change results in long disconnects. Sometime after waiting for 10 minutes ore more they reconnect, but at other times, connection can only be restored by resetting both devices by pressing their reset button. Then one has to repeat the whole setup of both devices again. Only to be disconnected again if you just change one settting.

Why aren't things properly explained in the documentation? Why offer bells and whistles, like clients being able to connect, when even the simple use case of extening wired ethernet can't even be accomplished?

Just create a brigde to which you can connect wired ethernet, that uses AC only (why bother with A and N), and just works. If you want customers to tweak settings, make sure they actually do something useful other than to break the connection.

Hardware looks well built, but firmware/software/UI and documentation are a shambles.