IMO, it's a case-by-case kind of thing. There are certain undead which are subject to spirit combat such as Ghouls, Thanatari Heads, Zombies or Skeletons with bound spirits animating them, revenants, mummies created through divine intervention, etc. Basically, anything which has a spirt even if they "do not have POW" according to the RAW rules. Not sure (have not made up my mind) about vampires and demons. Anything without a 'personality' is just a magical construct (unless animated by a spirit) and are not subject to spirit combat.