DoomWiki pointed me towards
this interview:
A lot of people found Plutonia much too hard for them (Archviles in the first level, etc) and so stuck to Evilution. What's your reaction to that?
Plutonia was always meant for people who had finished Doom2 on hard and were looking for a new challenge. I always played through the level I had made on hard, and if I could beat it too easily, I made it harder, so it was a challenge for me. I don't have a lot of sympathy for someone who plays Plutonia on hard skill and complains it's too hard. I had a lot of mail from people who had never even tried the easy skill setting because they "only play on hard". However, if someone does play Plutonia on easy and still finds it too hard, play Evilution through, and you should be ready to play Plutonia...
Also
this:
Plutonia does contain a few maps that were excluded from TNT... The maps were part of TNT until it was agreed to be sold to ID, then existing concerns about difficulty lead to their removal. Milo and I didn't want to reduce the difficulty because we were in a different mindset than most of the rest of the TNT team. We were more excited about providing a challenge and weren't worried about going against established conventions.
