Giuliano and Simonetta were pretty revolting...

While it's common for fictional portrayals of them being lovers, there is zero evidence of it. Only that both brothers admired her beauty (along with everyone else) and I found it absurd that they tried to make her husband into some appalling villain and even fictionally had him among the assassins in order to generate sympathy for the couple. Guliano is attracted to her for the most shallow reasons, seduces her after her husband generously grants her a lot of liberty to pose and we're supposed to be outraged that he's upset with her and pretend that Guiliano didn't destroy her life.

They had to gloss over certain facts like that he was deeply involved with another mistress who gave birth to a son shorty after his death who then became Pope and was supposedly madly in love with her.

Just like to make the de Medici sympathetic, they had to gloss over a lot of what they did to the Pazzi family. About a year or so before the attack, the other brother of Guglielmo and Francesco, Giovanni, that the series excluded entirely (I guess because he wasn't a conspirator and would be too sympathetic) had married the daughter of and sole heiress to the richest man in Florence and when he died, the very day, the de Medici used their influence to pass a law to exclude women in favor of male heirs in order to deprive a member of the Pazzi family from having such a large fortune. That's really more than anything what began the conspiracy. It was such injustice that afterwards Giuliano left Florence for a time after quarreling with his brother over the issue.