I used to work in ad-tech and this is exactly right. We had a lot of problems that caused a considerable amount of noise on the results:

• Cookie trackers getting added to the main blocklist for ad blockers basically rendered them useless
• IP tracking was basically done by matching IPv4 addresses which are not permanent. Apple launched a feature which made every person who had ever paid for an iCloud subscription resolve to the same IP in any given country.
• People generate so much data that it’s actually hard to try to tell a meaningful “story” out of it. Like GP is saying 90% of a marketing budget is wasted but it’s hard to tell which 90% that is.

I also think “attribution” is fundamentally flawed. Like the Coca Cola ads they show at Christmas are the most successful ad campaign ever but no one goes on their website after watching one of them.