This may be related to bug #522176 The xhci_pci module is required for USBv3-only computers to boot the Gentoo minimal ISO from a USB drive. Currently it is not being automatically pulled into the initramfs by genkernel and thus isn't available to find the root filesystem during the boot process.
Created attachment 406336 [details, diff] Adds xhci_pci to defaults/modules_load
*** This bug has been marked as a duplicate of bug 522176 ***
This is actually not a dupe, since the other bug was about the kernel config that has now been updated. I'm going to push this change to the genkernel repo.
https://gitweb.gentoo.org/proj/genkernel.git/commit/?id=f6a2b607a579ce37048fba34cea9dcd28548014c