Your reply regarding using ANY label for rootfs in Uboot compare to Ubit using just rootfs hardcoded label doesn't address the issue that ArchLinux assigns /dev/sda1 always to SATA drive if present, even without rootfs and regardless of any label. It simply can't use the LABEL param supplied by Uboot. Because ArchLinux on a PC can be booted by various bootloaders which can deal with booting from a proper drive themselves with Mobo support, while ARM devices are booted by Uboot which also needs Mobo and Arch cooperation for that, technically it may look like a bug of ArchLinux ARM or the stage0/1 board boot logic/code. But I don't know, if anyone here is capable of fixing it.
For example, I run ArchLinux on a mutiboot laptop, and boot it via Grub4DOS having no issues of booting from USB disk provided USB boot is supported by the laptop Mobo. In fact can boot Arch via Plop from USB thumb or disk after booting to Grub4DOS on SATA drive, or select in Grub4DOS menu any other device to boot from. But for ARM devices, I'm not sure, on what stage /dev/sda1 is assigned to SATA when connected - it may be hardcoded, and why there is no check if the SATA drive has rootfs at all under any label, and what bootloader params are.
However, the fact that Debian can boot from USB on GFN via uInitrd while SATA device is present shows, its possible to fix the issues in ArchLinux ARM. The problem is, any bug reports I posted here were closed by the devs instead of fixing them. So it looks like a utopia movie: everything is always good here. They just jump on folks like me reporting the bugs. I don't think its good for business either, as customers aren't stupid, and Linux upstream always has numerous reported bugs as work in progress.