I tried to NAND restore an older image under beta 3 on a C3200 and it failed. So I lost my installation completely. There was normal NAND restore operation, but I couldn'd boot the system afterwards (kernel panic). The used CF card worked perfectly afterwards in other devices.
I think, it depends on the CF card you use. If I have to, I will try with a new CF card next time. I never had these problems with my old C860, but I think, no one should trust a NAND backup on a Cxxxx without trying it with a specific card.