I have the same concern. Nudging is an umbrella term for a vast number of very different activities. For example, nudging is a term used for motivating more carefully designed road markings. I find it hard to believe that some of these newer designs don't "work" better than the old ones, some of them are quite ingenious. At the same time "nudging" is also used for all sorts of public policy framing issues that are more questionable and have probably harder to measure effects. As you say, each "nudge" needs to be evaluated individually.