There are people who can't play the original IWADS on UV so of course to many Plutonia seems like an insurmountable evil. We have to understand that from the perspective of a normal gamer that has a casual relationship with Doom and has never touched anything the community has produced, Plutonia is going to seem brutal because it is definitely a large spike in difficulty over Doom and Doom II. A similar thing happens when people are playing Ultimate Doom for the first time and are unexpectedly slapped in the face with Thy Flesh Consumed.
If you have eaten, drank, slept and shit Doom for decades now of course Plutonia isn't going to have that same challenge anymore. When people talk about Plutonia's difficulty it's not a discussion aimed at this audience.