There’s an argument to be made that it is abusive by simply existing. It’s a business that is built on network effects since it is a multi-sided auction (people offering ad space and people bidding to show ads in those spaces). There are massive barriers to entering the ad network space as a result, which lets incumbents get away with whatever they want. In my opinion, markets that are inherently not competitive, like ones reliant on network effects, need regulation and forced competition.

But more specifically, it is things like Google forcing site owners to enter anti-competitive agreements (see https://curia.europa.eu/juris/document/document.jsf?text=&do...). As part of the agreements Google required, site owners had to preserve the best ad space on pages for Google’s ads instead of ads from rival networks, they were not allowed to insert ads from rival ad networks on the pages that Google search results link to, and they had to get permission from Google before changing how rival ads were displayed on pages where both existed (allegedly).

Note that even if site owners found these clauses problematic, in a practical sense they have no choice but to just say “yes”. They need ad revenues to survive in today’s economic environment. And for them, the risk of not having ad revenue is existential while Google doesn’t have anything to lose by being aggressive. It’s an uneven situation where there isn’t really choice for anyone except Google.

Too many ways to count, really. It's an enormous system and I'm not qualified to explain it in detail, nor am I a customer of their products.

The long-and-short of it is that Google wields undue monopoly power by mediating their competitors in online advertising, and abuses that control to manipulate ad rankings and kneecap paying advertisers. They do this in several ways, like changing the font/frame/location of the advertisement and fixing it's ranking relative to Google's own products. This is the main argument against them, though there are tons of little inconsistencies that many highlight as salient.