Pros : The long opera sequence, the Vatican subplot, having Raf Vallone, the little nod to Enzo the baker, Al Neri still there, the landscapes in Sicily. Not ruining the Tom Hagen arc and character with a dumb, pedestrian rivalry and betrayal story.

Cons : the soapy love story between the two cousins, the inexplicable change in personnality for Michael, Connie and little Anthony. Yes people change in 14 years, but you cannot jump from how things were at the end of Part 2, do a 180 degree and not explain anything. Not having Tom Hagen, a pilar in this saga, not doing anything worthwhile with Tom's son, the priest.

They should have reunited the team and do a new non-Corleone mafia story instead.
That way, FFC could have indulged his sister and give her a strong matriarch role, without betraying the characters. I get she was fed up with playing long suffering types, but the Michael-Connie dynamics were unlikely.
What is more, they then would have been able to invite James Caan to the party.