The Problem with Senna

There aren't enough hybrid items in the game, and any hybrid items in the game aren't optimized to her playstyle except maybe Hextech Gunblade.

If you build AD on her, you'll deal a lot of damage but you won't have much healing or shielding. If you build AP on her, you'll have nice heals and shields but won't deal a lot of damage.

I get the idea of a tradeoff, but it makes it very suboptimal in a support role where you don't get much gold, especially when AP supports don't suffer from this tradeoff (Janna, Nami, Yuumi, Lulu). Most ardent supports deal a nice amount of damage while still having huge shields and heals.

Either more hybrid items need to be added to the game, or she needs better ratios.

EDIT: For further clarification, I am aware her heal scales off of AD. The ratio is a joke though, hence me saying "she needs better ratios".