I'm fine with Fiddlesticks being a demon. I'm even fine with him being an unknown effigy in terms of how he relates to whatever horrors control him, if something does. I'm even fine with him being actively possessed.
What I'm not a fan of is the arms in his chest taking directive and priority over a lot of his actions like his running animation and his auto-attacks. That detracts heavily from Fiddlesticks as a scary scarecrow, and shifts him towards being little more than a demonic Gundam. Adding this other "body" that is so pervasive in how Fiddlesticks is animated just pulls too much away from his core design, and makes the scarecrow body start to feel added on, instead of the focus as it should be. Fiddlesticks is the scarecrow, one way or the other, and having the arms pop out so constantly doesn't make the scarecrow feel sufficiently incorporated into who he's supposed to be.
Plus, it feels like a bit of a copy of Senna's specters also reach and grab constantly with arms.