Functional, but UX desperately needs work on WatchOS
On WatchOS, the user interface design has a number of major inconsistencies with standard UI patterns:
The watch app is organized as a single screen with two trays at the top and bottom. The trays are accessed exclusively via touch. The stack is in the top tray and advanced functions are in the bottom tray. This is quite different from most software for the Watch, which generally use sliding cards to access things which don't fit on one screen.
The lower tray, sensibly enough, is accessed by swiping upwards anywhere on the bottom edge of the display and dismissed by swiping downwards.
The upper tray, on the other hand, is accessed by swiping down from the right half of the top edge of the display. There does not appear to be any function assigned to the left half of the display. The tray is dismissed by swiping upwards, but a full row of space in the stack is dedicated to a large "back" button which duplicates this for no discernible reason.
This arrangement simply isn't very good. Every Watch includes a hardware control for vertical movement and it's not used in any way despite the use of vertical movement for navigation. The touch controls are implemented strangely. The end result feels very non-native, is fiddly to use, and generally lacks polish.