Can't comment on Redox as I'm not familiar with it (maybe xobs is), but "for embedded devices" means design choices are made to accommodate smaller memory footprints - hundred k's of RAM, ROM; not the gigabytes expected in desktop-class OSes. So, this is in the same class as e.g. zephyr, threadx, chibi-os, Tock, etc. and has no explicit aspirations to be able to run e.g. server workloads. An example of such a trade-off is sticking with a 32-bit pointer size from the get-go. No desktop or server-class OS could make that trade-off, but the memory savings from smaller pointer and object sizes is meaningful on a memory-constrained device.