Maybe I'm just missing something here, but I noticed (what I see as) a huge flaw.
Under the fireball skill, there's two upgrades - Impact and Fireball.
When Impact is unlocked, a charged fireball will knock down enemies and briefly stun foes.
And with Scorch, a charged fireball will reduce targets chance to hit by 25% for 5 seconds.

So is there any benefit to getting both Impact and Scorch? I figure if you've unlocked them both and they happen simultaneously, then by the time that they get back up, the benefits of Scorch have already worn off. Seems like a waste to get both, in my opinion, but I'm happily open to any corrections.