Incredible book so what the Hell happened here ?

I can honestly say I feel that The Sicilian is Puzos best novel outside The Godfather. How did south get lost in the screenplay?
For the sake of steering clear of The Godfather franchise I understand not tying in michael corleone and peter clemenza but they left out so many key elements and added a bunch of garbage..
Stephan Andolini was completely left out..
The widow la banera was also excluded from the screenplay...this was such a rich book and they really left so much key elements out were they added some of the worst storylines I could of ever imagined
The dutchess part was expanded upon and just awful...
She throws water in the dons face in public and he laughs it off ??? She'd be a corpse...
Then the real foolishness begins...
The dancing scene ... Omg that was embarrassing ...
One of Puzos best work was hacked to pieces..
I'm not sure were the blame falls whether it's the writers or Cimino but I'm guessing both along with the studio...
This is a tragic ending to a near flawless novel my only hope is that one day this will be remade correctly in my lifetime..
What a waste