I definitely disagree that FFINO did not do the same disservice to the characters. I assure you that it dropped the ball in properly representing any character in nearly every regard, Doom especially. He's one of my favorite villains in all of comics and it's borderline pathetic that they haven't been able to quite get it yet.
However, I feel like were are approaching this question from similar angles. The visceral reaction you describe for Superman Returns doesn't sound too dissimilar from what I experienced with FFINO. I guess when you have a property so close to your heart and you see it mis-represented on screen it makes the whole thing seem that much worse. I still regard Superman Returns to be a good film, but I can understand why it didn't click with so many people.
I guess how I approached FF is that I totally forgot that there was an amazing comic called Fantastic Four ( I was, and still am a huge fan of John Byrne's work on the book in the 80's, he really took it places), that it's characters (both heroes and villains) are iconic parts of Marvel's mythology - and they can claim the title of "greatest superhero family" pretty handily.
Anyway, I totally forgot all that and gave the FF film a chance.
I guess I just couldn't do that with Superman Returns - mostly because it wasn't an origin story, but a continuation of the adventures of my all time favourite comic book character - and it left me with a terrible taste in my mouth.
Dude, if you felt as bad about FF as I did about Superman Returns then I feel sorry for you - especially if you paid money to see it on the big screen. I suppose one of the reasons I cut FF some slack is that I saw it on DVD ( it cost me $ 2 ), but if I'd forked out the cash to see it in the cinema, I would feel ripped off.
In all honesty SR is probably a better film, at least it's got a story that sort of makes sense, although it's a ****** story IMO - whereas FF doesn't really make much sense at all.
As for the characters, it didn't do them particularly good service (although I don't feel they fared as badly as Superman did in SR) I thought Reed Richards didn't come off too badly, but Ben Grimm got very little attention, as did Sue. Sadly, Dr Doom has never been done justice in a live action film - I preferred the 2015 version of Doom, which was pretty average, to Julian McMahon which was utterly dreadful.
One day somebody will get Dr Doom right - I mean you wouldn't think it would be too hard to get him wrong, he's the archetypal somewhat tragic, evil-genius, noble-enemy who wears a mask. How hard is that ?
Although having said that Lex Luthor doesn't seem to be much of a challenge ( the animated versions of him have been spot-on) but in the films he's pretty awful.
Ah well. Sooner or later a decent FF film will come along.