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Don't know the limit, I would guess the weak part is more the mount point on a fork, for example to mount it onto a carbon fork, which is more a limit than the rack itself. But I really don't know.
The difference between the M18 is weight and build quality, there's somehwere on the internet a difference shown between the standard Nitto racks and Rene Herse / Compass racks regarding the quality and everything. The struts are changeable, so Nitto and Rene Herse racks can be used for canti, or directly on the fork.
totally diff rack