Aura's feel like something that should absolutely be in the game, it incentivizes not getting hit and adds a ton of variance to gameplay.
My main experience so far has been with the Thorn Aura (Crystals -> Thorns, in case that's not what it's called). That was incredible amounts of fun and I picked it up when I had 1 crystal and 1 heart, and it carried me all the way through the second biome in a terrifying glass cannon fashion. If I had more health when I picked it up I would've felt like a god. It's definitely overpowered at the moment, in my opinion. Maybe make it truly a glass cannon item where you cannot gain HP while you have it?
I also came across the aura that's supposed to give you two free seeds, I believe, but I was confused about how to use it, if I needed to get hit? I ended up dying immediately so I didn't get to find out.