In my experiece, upgrading the kernel to a newer one is not a problem. What can happen, though, is that an upgrade of userspace will suddenly require a kernel-feature not present, if you run an ancient board-support-package with a 3.0-kernel or so in 2014! (happened to me with a BSP for a Freescale i.MX6).
So, if you have archlinux tracking the latest and greatest systemd, uptrading userland can make your machine no longer bring up everything (completely). But in my case the kernel is from 2011...
Upgrading the kernel to a newer one isn't generally a problem, but the changes to cgroups that the systemd and kernel devs are planning to make in the future are not going to be backwards compatible with old userland code.
After putting a current arch-linux rootfs[1] on the box[2], it seems to come up, just network and console seems to be misconfigured, but this, I think, I had to fix the last time also. So: indeed the current systemd seems to work on a iMX6 running a 3.0 kernel.
Recent systemd doesn't seem to realize that ethernet and the serial console device are actually there, it's waiting for dev-ttymxc3.device and sys-subsystem-net-devices-eth0.device ... but both are statically compiled in. So there is some issue with kernel 3.0.x.
So, if you have archlinux tracking the latest and greatest systemd, uptrading userland can make your machine no longer bring up everything (completely). But in my case the kernel is from 2011...