I was able to move to netplan on ubuntu bionic with own yaml files for eth0 and wlan0, by hiding the fenix-default.yaml file, because I use the Edge-V as an headless server and NetworkManager is used mainly in X11 environments.
I tried to completely stop, disable and mask NetworkManager with
In my very special case I am not using NetworkManager while running in headless mode and using netplan and systemd-networkd is enough. I have a good experience with netplan using Ubuntu Core 16 since two years.
My mistake was to only mask the NetworkManager service. This is not sufficient.
I had to remove it completely with sudo apt remove --purge network-manager from the system. After a reboot WiFi was accessable over systemd-networkd