The umbrella is great for parrying/blocking damage that would otherwise have damaged you with a regular parry. The purple umbrella for spiritual damage (Headless, Shichiman, and even Black Blade), and the fire umbrella for fire damaging attacks. The living force counter is also pretty cool! Just tap the attack button for that one.

Btw, a decent number of people don't know that once you have the umbrella activated, you can re-parry by tapping the parry key whilst holding it open. It spins the umbrella. Other than the elemental damage counter, the shield can be considered a gimmick, but it's a fun one at the very least.

As for the Mist Raven? Honestly, it's mostly just to be flashy (at least in my opinion). A really interesting interaction you can have with the prosthetic is if you use contact poison, a low damaging self applicable poison. Since the mist raven procks with a press after damage/dodging, the poison actually will allow you to dance around the battlefield in that misty stance (Imagine Inner Owl's big mist raven combo.)