Why is Diana so overpowered?

[Yes I recently lost to a Diana]

Recently, I have NEVER been able to beat a team that has Diana. Why is that? It's because she has the burst of a Leblanc, the power to tank half [or all] of my team, and an ultimate that practically has a 7 second cooldown [or whatever her Q CD is because that skill shot is RIDICULOUSLY easy to hit].

Honestly, why don't they just make it so if she doesn't use her R a second time within 10 seconds, it'll go on cooldown? [And her Q would reduce the cooldown if it is on CD]

Atleast then she can't use her ulti 5+ times over the duration of one fight.