The initramfs images are the only ones safe to flash from oem.
You can't flash the wrong image on a ZyXEL switch since they use a trailer to validate the images, and we only add that trailer to the correct images.
This is a bit harder with other vendors where we don't have this safety net. But you will be fine as long as you make sure you use the model specific initramfs.
Note that you should do a sysupgrade after booting OpenWrt for the first time. The initramfs images cannot store any persistent config or additional packages. And also note the weird default network config of. the realtek target: "LAN" is defined as VLAN 100 on port 1. This has fooled most of us.... Everything else is "WAN" and therefore firewalled. So you can only access OpenWrt using a tagged VLAN 100 on port 1 until you have installed the sysupgrade image and configured networking as you want it. Very annoying, and something to be aware of when doing a console-less installation.