When using genkernel-next-64 for booting the identical system from a) external USB 3.0 mechanical harddisk or b) external USB 3.0 SSD (tested with Samsung T3 SSD) only a) works. After countless reboots I found the reason: genkernel-next does not include the kernel module "uas" in the initramfs by default, only usb-storage. Missing that the SSD is listed on the USB bus (lsusb) but no /dev/sdX is created for it, consequently he does not find the root partition and fails. Inclusion of "uas" module in the initramfs makes boot from a USB SSD work.
Package removed.