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.
Can you give an example of the "no longer bring up everything (completely)"?