Coming here to say the same thing. Just installed SteerMouse and all horizontal scrolling is ignored with both BTT and SteerMouse are running. In my case, it was just the shift key that was being ignored.
With BTT running and setting the horizontal scroll on a Command key modifier SteerMouse was able to pass through.
Even without any mouse events being captured in BTT (I deleted all of them), of which I had some in both FinalCut and Sketch apps, SteerMouse was being preempted by BTT.
Quit BTT and SteerMouse works, run BTT and horizontal scrolling with shift stops. I agree that something is going on between the two apps.
My solution was to disable BTT for the dedicated apps where I prefer SteerMouse. The solution is here.
The downside is losing mouse support in all BTT for the sake of horizontal scrolling via SteerMouse? Further testing needed.