Thank you, @Patrick, I wasn't aware of the different Quaternion definition in Unity. I applied this and everything looks a lot more like it should.
Now, the rotation looks fine, however the spatials z-Axis (Unity Y) is always lerping back to a value between 200-235 degrees (euler) when I hold the device still. The only lerping in the code can be found in ComplementaryAHRS.scaleQuaternion, but due to my lack in proper understanding of quaternions I cannot quite find out how to fix this.
Do you know why this might be happening?