Running--comically inaccurate heart-rate info. If I want to know my lap time while running up a hill, I have to wake the screen up, then hit it twice, then the watch tells me in a huge font my "pace" (which is so inaccurate as to be worthless) and while the actual lap time is displayed, it's in impossible to read dark, 3-point font.
Swimming--tbf, some improvement with adding "last lap time" to display, but the inability to have display be always on (i.e., full brightness, so it's visible underwater) still makes the watch more like a gimmick than a helpful training aid.
Worst is recent change to Samsung Health "report" for swimming which feels like it was designed by a Martian. Used to be intuitive bar chart to summarize lap times, now it's changed to messy table where 1/5 of the screen is taken up by telling you the length of each lap (you know, in case the length of the pool changes while you're swimming or something.)