Chain checkers are valuable because they reduce human error--they are less precise than a literal measuring of length of a tensioned chain can potentially be, but in a shop environment in particular mechanics should check chains on literally every used bike they touch, and speeding up that process is critical. Measuring with a ruler accurately takes a steady hand, a good eye, and a solid moment of concentration. Your Pedro's tool, along with the Shimano chain checkers, are the most accurate.

At work I use a Rohloff chain checker. I've never seen it underestimate wear, but the majority of the time its results are sufficiently accurate. I do double check with a ruler when the result is borderline, particularly when considering whether a cassette of freewheel is likely to need replacement due to excessive wear.