At the highest level / difficulty, the only career worth playing is pyromancer. Here is a list of things that I think are holding back the other careers.
The level 25 talents that allow you to vent all overcharge on skill use. The cooldown on pyromancer is only 40 seconds, which allows for unlimited spell casting. Compare to the other two ability cooldowns, which are 2-4 times as long.
The battle wizard and unchained ultimate abilities are much more niche in when they are useful. The Pyromancer ability is always useful, and almost always available when you need it.
The battle wizard passive that grants charge speed isn’t even noticeable.
losing tranquility upon taking damage means it is almost never active (disabled from gas rats, being pushed by elites, leech teleports, etc.)
Those are my main concerns, as well as talents that seem out of place or have no practical use. I love Sienna but there is really only one career right now.