I use an indoor rower and have been trying to use the Indoor Rowing activity on the Vivoactive 4 to track it for the past few months, but it's only counting about 1/4 of the strokes, on average. It simply doesn't recognize most of the strokes. A normal behavior is I can row for 20 or 30 strokes, and it won't count any of them, then it will count the next 5 or 6 in a row, then it won't count the next 15, etc. It goes on and on like this, like it's random. The highest I've seen counted is about 60%, the lowest about 15%.
The step counter works perfectly and floor counter is adequate, so I don't think it's the accelerometer. Something else is going on.
I wish it would just use a cadence sensor to count the strokes (the one I have on my bike works great) instead of trying to count on its own. That would be so much more reliable.