Why doesn't spline IK work with arms?

I’ve been experimenting with IK setups and I have a spline IK down the spine that I use to twist the spine. This spline IK works fine. When I try to assign a spline IK from the Left shoulder to the Left Hand in the same fashion I get some serious oddities. I’ve tried the various axis and configurations and none work the way the spline ik does down the spine. I’m testing using the mixamo character pack (goblin). Is there a certain way that spline IK prefers the bones to be? Is it because the spine is symmetrical up the middle axis?