iXSystems announced that the RAM limitations on Linux for ZFS ARC will be going away in the next release. Other than that I think if some of the last major BSD developers are choosing to move away then they probably are doing it confidently. It's not like there aren't Linux systems running on iot microcontrollers. So you can't say that Linux can't scale down. In fact most small NAS systems on the market run Linux, I don't know of a single commercial 2-4GB RAM NAS running BSD except for the ix mini.
Worrying about the pace of kernel updates on Linux seems to be somewhat moot as well since iXSystems controls their distro so they can pick and choose the pace that Scale adopts updates.