I'm going to stick my neck out & admit that I actually enjoyed both the Winegardner books as well.

I've been re-reading them, simply for some brain-candy in between reading serious novels, & my original opinion hasn't changed - they're utter trash, but they're fun, simply because it's more material about my favourite fictional family of all time.

However, I can understand the overwhelming negative reaction from hardcore fans, & upon re-reading I found a few arrogant things I'd forgotten about, particularly the changing of Kay's abortion to a miscarriage. It's one thing to take a less-than-obsessive stance on keeping trivial details accurate, it's another to reverse one of the most powerful actions a main character made in the originals.

The biggest problem with both novels is Nick Geraci - he gets more & more irritating as the story progresses. Not because he's an obnoxious character, it's because he's transformed from a vaguely interesting new character into Super-Don, is promoted to the head of the Corleone Family, then, to emphasise his cerebral side, just happens to develop a love of jazz & starts writing a novel while on the run. Is there anything he can't do? Lucky he gets knocked off at the end, otherwise he'd have taken the Mafia into space exploration by the time of GF Part III, & cloning by the turn of the century.

Now I'm really going to piss some people off - I didn't mind the twist of making Fredo gay at all. It actually worked. Nothing against gays in general, but John Cazale's awkward, weak portrayal of Fredo does (in hindsight) fit the character of someone both hiding & repressing his sexuality in an environment that would never, ever accept it. It also ties in with his miserable marital situation with Deanna Dunn as portrayed in Part II - she makes a crack about wanting "a real man", after all.
It's also a subplot treated without political correctness, as it should be - Fredo truly hates himself for his desires, as if he's diseased, & that's exactly how someone with his background, in that situation, in that era, would feel. One could imagine the concept being treated with 'sensitivity' & 'compassion' in the hands of some writers - & it would come across as total bullshit. Instead it feels like the real deal, & heightens the pity one feels for poor old Fredo, who never really wanted to do any harm to anyone.

Anyway, that's my two bucks, I could pick apart every flaw in both the novels, but I still enjoyed them. The mythological status the GF films have taken on seems to have overshadowed the fact that Puzo's original wasn't a masterpiece either, it was a pulp novel, nothing more, with some very sleazy stuff in there among the nuggets of brilliance.