Safe to operate with arbitrary N,L connection?

[Thomas R.]

I know the Plus 1 works even with L and N swapped, but it is safe to use this way? I intend to integrate it into a device connected to a german wall socket.

[thgoebel]

It’s save, Don’t worry!

[Pluto]

Think about it this way: a regular power plug without ground, e.g. from a desk lamp, could be plugged either way in a socket and it will still work. The in-line switch will also still work even though in 1 scenario it would be cutting Neutral instead of Live. The only risk you have then is that the Live end of the bulb socket would still be hot even when the lamp is supposedly turned off.
A Shelly in a Shelly Button housing replacing the in-line switch of said lamp could thus also be wired either orientation.

Unless there's building code mandating where to put which phase (L or N) on a socket you wouldn't even really know how to correctly plug in the lamp's plug anyway.
So the same is true for Shelly Plug devices.

So your Shelly would function regardless of whether you adhered to L/N or swapped them.
It just makes sense to follow the markings where possible. If you installed them behind a socket or switch where you directly wire it into the circuit, stick to the proper wiring. It will make the live of future-you much easier ;)

Do be aware that for the Shelly's without dry relais, L is used to feed O. So if you were to wire N to L, the outputs would provide Neutral instead of Live.