I ran into the same issue. I had copied code from IOUserHIDEventDriver.cpp which doesn't call RegisterService() - even though it says "An IOUserHIDEventDriver object is a fully functional driver..." on the docs page. Adding the RegisterService call fixed my issue too. Better docs are needed.

By sampling WindowServer, I found that IOHIDPointerScrollFilter is being loaded from /System/Library/HIDPlugins/IOHIDPointerScrollFilter.plugin. I wonder if my plugin would load from here. I'm guessing its more difficult.

In this talk it is also mentioned that you can use Core Foundation in DriverKit...

For me the fix was to change the hub that my cable was plugged into. Every time I would try to sync my iPhone would disappear from showing up in Finder.