The fact that Google (and others like FB) have not the same incentives as the company buying the ad-space. Even more, I kinda have to trust Google that they show the ads to as many (real) people as they say. Remember Facebook and the video views metrics [0]? Even more so, I have to trust them that they don't manipulate my ranking after not buying ad space anymore to incentivise me to buy ads again.

If a company has enough budget and it's more about making sure that as many people as possible learn about a product or company, Google Ads may be useful. For niche companies without the funding, I'm not so sure...

[0](https://www.theverge.com/2018/10/17/17989712/facebook-inaccu...)