Why systemd and udev Affect Device Application Startup
On embedded Linux devices, applications often fail only during boot: a serial port has not appeared, the NIC has no address yet, the data partition is not mounted, the camera node is missing, or permissions have not been applied.
The common patch is:
ExecStartPre=/bin/sleep 5
It may appear to work, but it turns dependencies into a timing guess. If storage enumeration is slower, DHCP takes longer, or a driver is probed later after -EPROBE_DEFER, the failure comes back.