First you create a horizontally scrollable galleries. Then you decide to add swipe navigation. As a result, when I swipe, one of two things might happen: I either scroll to the next photo or switch to another screen. There’s about 50% chance on _every_ interaction to get not what you wanted. Pretty bad.
Another downside of scroll navigation is that if you’re scrolling not perfectly vertical or your finger accidentally moved during the _vertical_ scroll, you might get punished for that with unwanted screen switch.