I had remembered reading about companies/advertisers suggesting that the extra $$ dumped into targeted ads on FB resulted in no extra revenue they could tie to that campaign. Sorry, I know that without links it's just a vague recollection that could be wrong.
Obviously this question has no bearing at all on whether or not personal data collection and mining has value (it does, even if not monetarily - e.g. at the state level).
But when it comes to Internet ads, I find it's the opposite. Take Youtube for example. It is often the case where Youtube will show me the same 2 or 3 ads over and over for quite a few weeks. That is extremely annoying, and makes me want to never ever have to deal with those brands. Seriously, fuck those two insurance companies, or that macho soap brand, also Google Pay, or that AOE clone game. It is extremely annoying. I know exposure to the brand leads you to become familiar to it so more likely to buy it, but the kind of extreme exposure that some internet (targeted) advertisement makes me associate those brands with negative feelings. That can't be good for their brand.
What you'd like to look for is those results proving it doesn't work. Personally, ads are simply an annoyance and sometimes even hostile at times.
I am not expert in the field but I am sure that there are hard data from studies available on the subject.
A question is whether it is worth paying the extra money to run a specific campaign on a specific platform and the answer is, as often, "it depends".