> Perhaps Google does well for their publishers but do they do well for advertisers? Inherently it seems like it's impossible to do both because what's good for one group is bad for another.
There's certainly some tension between advertisers and publishers, in that advertiser would like to pay less and publishers would like to be paid more; but there's a lot of things an ad exchange can do that are good for both. Selecting ads to display that result in meaningful downstream conversion is good for both advertisers and publishers, because they'll both get paid and maybe something about the user getting something they want too.
Showing inappropriate and ineffective ads isn't great for the advertiser, and it might make the publisher money in the near term, but it can drive users away and tends not to be sustainable --- advertisers stop advertising in venues where they don't get results.
The value of a good ad exchange for the publisher and the advertiser is when it provides reasonable matching at a lower cost than the parties arranging advertising directly. Possibly some amount of assurances for both sides too --- the exchange should ensure the advertising code and destinations aren't going to compromise the publisher or their user and should ensure that the ads paid for are actually seen (to the degree possible). There's room for the exchange to profit from scale while still being lower cost than self-managed advertising.