Hello,
Recently I decided to give iwd a shot because I have several Pi zero W projects and iwd is supposed to be less demanding on resources.
I tested on a RPi3 where I use netctl and wpa_supplicant. Disabling those and using iwctl to setup a fixed IP address (all my Pi's have fixed IP addresses) the wifi works fine. For ethernet connection I already had dhcpcd setup for a fixed IP address so connecting and disconnecting the ethernet cable automatically switched from ethernet to wifi and the other way around. Perfect.
Then I installed and configured iwd on a RPi0W, same process as on the RPi3. At first wifi looked fine but then I noticed the date was way off. Running systemctl status systemd-timesyncd shows that all (default) time servers got timed out so the system clock never synchronizes. This was on internal wifi but disabling this in /boot/config.txt and using external wifi adapter I got the same result. I even tried different adapters. Doing a pacman -Syu works fine. Everything works except for the time synchronization.
On the RPi3 systemd-timesyncd is synchronizing immediately at the first time server (0.arch.pool.ntp.org). On the RPi0W when connected using netctl, systemd-timesyncd is synchronizing and the ping 0.arch.pool.ntp.org response time is the same as when connected using iwd. I also manually set the time using the date command and restarting systemd-timesyncd still times out. I also tried different time servers, same result. I even did a fresh install on a new sd card and tried on 2 different RPi0W's, same result.
The only difference I can think of is that the RPi0W is armv6h instead of armv7h (RPi3). I read that iwd is utilizing kernel features so could there be an issue?