Uh. I used Android for 15 years and I still keep guessing wrong what the back button will do. Feels like 75% of the time it does not lead me back to what I expect and all apps have different ideas what they think it should do. I agree though that not having a back button at all is also not good.

Apps can have questionable Back behaviors but they all have a unified way of invoking Back.

Apps on iOS can also have questionable Back behaviors, but the method for invoking Back is not consistent.