It was next to useless on my S4 before I ditched Apple Watch. Even with an absolutely rigid overnight docking schedule, it would always fully charge at least four hours before I needed it. On top of that, if I ever had the temerity to alter my sleeping schedule by ten seconds, it would have a panic attack and then take weeks to re-enable itself.
I actually find this pretty typical of all Apple devices that support this feature. In my opinion it’s almost pointless based on my experiences with it. AlDente for Mac gets it right by allowing you to set your own limits and parameters, but Apple will never let its users think for themselves because, well, Apple 🍎