Those "pocket" gauges aren't very reliable.
I used to compete in 1000 yard target rifle. A few guys tried those gauges, but found that "reading" the flags was better. Plus, they estimate the wind right at the instrument, but the wind can be different 100 yards away. We had flags every 100 yards down the range (sometimes blowing different directions and strengths).
But are they reliable compared to a traditional (single) track&field wind gauge?