Three things about Fiona that I had a hard time with
One...The way she was so violent with Michael, always hauling off and slugging him. I mean with his background of abuse, that seems like a bit of questionable writing (Maddie too), but then again, maybe it's realistic...people tend to be attracted to dysfunctional relationships based on their past relationships, and one thing's for sure, Fi & Michael have a dysfunctional relationship.
Two...She never seemed to be violent with her other boyfriends, never saw her haul off and hit Campbell or Carlos. She seemed quite reasonable and tender with them. It's hard to believe if her nature is that violent, that she wouldn't behave that way with them too. (or at least let them see how crazy she could be.)
Three...How she fell in love with Carlos so quickly in season 7. And was genuinely crushed when he left her. I just had a hard time with that, after everything she and Michael had been through...I understand she was seriously hurt that he made a choice that she believed meant he didn't value her as much as his work, and then disappeared without a word. So she was trying to mend a broken heart by opening it to someone else...but it was still hard to take...it would have been easier to watch if they'd left out the love stuff. (of course, it was also a necessary plot device, so that Michael could sleep with Sonya.)
I do think that perhaps the Carlos Fi relationship existed ultimately to demonstrate the kinds of extreme things that Fi & Michael were willing to do for each other that any other regular love interest, like Carlos, just couldn't. In the end, only Michael & Fi really got each other in that way. They had the same level of commitment to each other, willing to break the law for each other, willing to die for each other... whether they were together or not.
I still wince every time I see Fiona or Maddie hit Michael though, but I know it makes for a more dramatic effect. And maybe it is a more realistic plot device than I first thought. Because basically, I think Matt Nix is a genius, and he wouldn't do it if it wasn't true to his vision.