The udisks dependency in xfce4-power-manager is only required to set spindown times for disks, which isn't really needed for SSD. udisks-1 also pulls in a lot of dependencies. I tested this by creating a local ebuild that removes the udisks dependency, and everything works, but the spin down checkbox is greyed out. A use flag description that says "required to spin down disks" would be nice too Reproducible: Always
You have a point with SSD and futhermore, udisks:0 is really deprecated in favour of udisks:2 So yeah, it's now optional in tree
