The Fiery Heart is almost entirely centered around Sydney and Adrian's relationship. So if you're someone that prefers less romance and more plot then this book probably won't be for you. I know it's important to show that their relationship has grown and that Sydney is ready to take it further than she thought she would, but sometimes it just got to be too much. They were constantly in a state of romantic bliss and that's nice for a little while, but an entire book of that just isn't for me.
The other characters are practically nonexistent for most of the book. Sydney and Adrian aren't the only two characters I like from this series, so I would have liked to see more of Jill and the others. There was an appearance from Rose and Dmitri, but that was also short because obviously this series isn't about them.
The ending was by far the best part because finally something happens to really move the plot forward. It isn't a surprise what happens to Sydney because it is pretty obvious that the author set everything up for this exact ending, but that didn't take too much away from the ending for me.
In the end, I would have liked more action and just a little less romance from this book, but I still enjoyed it.