I use a filter app myself on Android and it isn't perfect. System integration is always better for this sort of thing, since it gives more flexibility for getting a better outcome, and isn't reliant on bypassing system security or using strict APIs. The problem with Android apps (such as Screen Filter and Night Mode) is that they don't cover the navigation keys, so if you have a device like the LG G4 you still have a patch of screen that's bright, since these apps work by putting a transparent window over the screen, but are restricted when it comes to covering the navigation keys. If Google integrated a filter it would actually modify the system colours themselves, so not only would you have less risk of erratic battery drain but the UI would be a lot clearer.
I would liken this to tethering - you could get apps that enabled tethering on Android years ago, but they were never as reliable as the baked-in feature, once that arrived on the platform.
Not wanting to partake in some OS rivalry rant; was just making a point that in this instance a system feature is an advantage over an app.