What is the doctrine of spiritual monism in the Upanishads and what are some appealing and problematic aspects of it?
The proper term for spiritual monism is non-dualism or Advaita, which posits one reality and the existence of no other entity. So, there is no duality or dvaita, so non-dual or A-dvaita.
This is the principle one homogenous, conscious principle being the reality and all things being perceived due to ignorance. The problematic aspect is the ignorance is not seen as such and is confused with reality. Also, the unitary principle cannot be witnessed by any ego or individual as there would be nothing that would witness because then there would be no more a unitary reality.
With best wishes,