I'm running a gentoo system with raid + lvm and a separate /usr partition.
Using openRC everything works fine.
When using systemd the initramfs won't boot because the default linuxrc script would test the presence of "/dev" and "init" in the NEWROOT at a time the /usr partition has not been mounted yet (and "systemd" which replaces "init" resides in /usr).
I've tried modifying the linuxrc removing the "init" sanity check (ugly hack) and the system booted OK.
I've tried also to anticipate the /usr mount so that the test is OK and this worked also but I am not sure of any possible drawback of this on different configurations.
*** Bug 508686 has been marked as a duplicate of this bug. ***
*** This bug has been marked as a duplicate of bug 479730 ***