Another thing to be considered is the temperature of engine bleed air versus the output of a motor-driven cabin air compressor. Engine bleed air is hot to say the least and can decompose engine oils as well as vaporize it. The compression ratio of a motor-driven cabin compressor is far lower, hence output temps would be too and I would think its ability to break down the oil. Agreed, I do not want to breath undecomposed oil either..
APU bleed air and main engine bleed air can also decompose deicing glycol as I have seen where an APU was used to run the air conditioning during deicing and ingested deicing fluid. We had a return to the gate with a lot of nauseated pax.