There's the general risks that they have your data and in future can change what they do it. They can be bought out in future like fitbit, which is obviously laughable at the moment but it's not the first time we've seen apple sink. Plus there's the general bluetooth privacy issues, although I think apple has a decent track record of protecting against this.

I'd be interested to know what and how much it shares info too and if it's safe against things like wifi MitM attacks.