We tested this in our kitchen. A glass pyrex used as precisely as possible was off by more than 5% in repeated tests. Our kitchen scale was off by less than 1% for weights over 5g.
And honestly, I am comfortable just pouring the milk/water/vanilla directly into the bowl that is on the scale. No utensil to get dirty. I recognize that I could over pour and mess things up but it just doesn’t happen. I can hit 15g of vanilla more accurately with the scale than with a measuring spoon.
It is hard for me to believe the people who are pro-volume have really tried the weight method with a decent scale. I am so with you on things like honey, corn syrup, molasses. Those require both a volumetric measure and a scraping spatula and it is still so much harder than squeezing the bottle until you hit the right number of grams.