My issues with a blanket recommendation of fresh fruits and vegetables are around (1) perception as calorie-neutral, (2) prep time and (3) overall caloric variance.
1. Regarding perception, for me it's very easy imagining apples are "free" (= 0 calories) so I can just eat as many as I'd like. That's not true because an apple is approximately 70 calories. So in a universe where I think apples are calorie neutral (e.g. Weight Watchers has 0 PointsPlus apples) I might be overindulging in apples.
2. Prep time. Most vegetables require either a certain presentation or preparation. That time investment is a "barrier-to-eating" and could cause me to consume higher calorie foods and that's why I prefer very low prep time snacks. Nature's packaging requires some assembly in most cases.
3. The caloric variance amongst is meaningful. e.g. Celery being at 16 calories/100g and potatoes being at 77 calories/100g. That caloric variance there cautions me from saying carte-blanche "fresh fruits and vegetables work for my kind of diet".