https://support.google.com/google-ads/answer/6394265
Actually Google is preparing it as a product, so I would guess the result was generally positive? FB also has a similar product as well.
the google page you linked also says, "you won't be able to continue using this model if your data drops below 10,000 clicks on Google Search or below 400 conversions for the conversion action within 30 days." depends on what your market/CTA is, but a lot of SMB likely don't hit these thresholds. similarly with FB, my understanding is that you have to go through an account rep to set it up and that the minimum spend is fairly high to qualify.
i think that these measurements are possible to do with a reasonable level of certainty, but they require a particular set of circumstances to pull off.
mcdonalds is actually one of the savvier players here, they know that it's hard to quantify brand advertising if you only go off in-store purchases, so they spent very heavily to promote their app and develop a consumer-brand relationship there. now, if you see a mcdonalds ad on youtube, they can (try to) match your youtube ad impression to a purchase done through the app / credit card / other PII.
there certainly are a lot of numbers in adtech and marketing. whether or not those actually relate to causal impact, across the industry, is not definitively answered. very few players (on the supply and demand side) have the scale and ability to properly do incrementality testing.
personally, i think people don't want to know the real results. if the numbers say it's not broken, why risk your job?