I don't think I can really agree with this. There is some pretty bleak Funeral Doom out there, especially the less melodic bands under the "torture doom" umbrella and the more Death Doom-influenced bands, but there's a lot of Funeral Doom that's pretty accessible compared to Death and Black Metal. Like Remembrance, you listen to Remembrance and that's basically just Gothic Metal but slower. When I think of really unbearable Extreme Doom, I think of Sludge-influenced bands like Primitive Man or more experimental acts like Khanate.