To answer point 3, well, it's too experimental for some. Which is to say, they feel the experimentation has gone too far at the expense of listening enjoyment.
Another factor is that the band themselves think it is arguably the worst thing they ever did.
Here's Roger Waters:
"Atom Heart Mother is a good case, I think, for being thrown into the dustbin and never listened to by anyone ever again!... It was pretty kind of pompous, it wasn't really about anything."
And here's David Gilmour:
"I wouldn't dream of performing anything that embarrassed me. If somebody said to me now: "Right...here's a million pounds, go out and play 'Atom Heart Mother'", I'd say: "You must be fucking joking... I'm not playing that rubbish!". 'Cause then I really would be embarrassed."
So I think a lot of people would hear those sorts of comments and think "well, it must be rubbish then" and then listen to the album knowing in the back of their heads the creators don't even like it. Which would affect your judgement in some cases, I think.
Regardless, I think it is a brilliant and vital and essential album. That may have something to do with having heard it on lots of acid, hehehe.
With all that said, to answer point 2, I do think Pink Floyd, as much as I love them, are capable of screwing up. I think the early period stuff before Atom Heart Mother is hit and miss, particularly Umma Gumma which is unlistenable in places.
Also the very late period stuff is less than phenomenal. Every thing after The Wall. None of it is unlistenable, but it doesn't reach the heights of their golden period stuff.
