All three factors can affect results on their own. To complicate things, the three factors interact with one another as well. This is generally declining in importance as a common factor in performance problems. Most farmers recognize that herbicides cost enough money to warrant great care in proper timing and careful application. Most herbicides are applied on time unless environmental conditions keep farmers off the field as weeds advance in staging.
All herbicides have weeds that they are strong on, and weeds they are less strong on.
Take the Manitoba Agriculture ratings for several canola herbicides in the chart below. The chart ranks products based on consistency over a range of conditions. You'll notice that all products have a range of degree of consistency across weed spectrums. You'd expect then that adverse weather, or other stresses would affect lower rated weeds sooner than higher rated weeds. You'll also notice that the ratings vary widely from product to product.