I think it's the lack of single target big hitters, lack of control and the abundance of PBAoE moves that can get you squished in a team without good aggro control. It's the standard blaster problem of having the damage but not being able to use it, only worse. Putting two pbAoE auras there in addition to the 4 other pbAoEs seems needlessly cruel. 
/Fire is OK AoE damage, and allows you to get more pool powers as it doesn't have too many must-have powers. I'd only take it with fire primary though, because if you're going to make an AoE blaster you should have one that can defeat a group in one attack cycle, otherwise you're dead.
I've heard that the Fire secondary is really weak. But looking at the powers it has 2 PbAoEs ( which you probably need a tank to use), build up, an end recovery power and a few melee powers. So it seems okay as a damage dealer.
Is it considered weak because it doesn't really help you solo as it lacks control?
@Unthing ... Mostly on Union.