@oui I've tested NetBSD 9.0 (the img file nested inside the ISO), current (NetBSD-9.99.72-aarch64-install.img) and the image from Jared (invisible.ca = armbsd.org). Current is very different from the others, but NetBSD 9 and Jared's images have exactly the same layout.
This is why I suspect the aarch64 image is pre-configured for the Raspberry Pi and there is something to do to use it on another EVB. But I have no clue as to what I should do and the INSTALL.* files are somewhat cryptic and don't seem to refer to the downloaded image.
@pfr I have both an RPI 3 and an ODROID-C2, I've played a lot with them under Linux. There is a huge difference between them in terms of manufacturer commitment! RPI is very active but HardKernel has never supplied a Linux kernel more recent than 3.x.
It doesn't seem so important until you want to use an USB device unfortunately unsupported in kernel 3.x. :( Or until you want to use it with another distribution (only Ubuntu and Android are officially supported).
If you intend to use your EVB with NetBSD, it doesn't matter much, you already know not all the features of your system will be usable.
However, if you have some project in mind that will make use of features supported under Linux, you may want to consider manufacturer commitment.
At the time I bought my EVBs, Pine A64 was not available yet. Today, I'd consider one of their boards very seriously. :)