I once was asked to write a feature story about Lladro, makers of ceramic figurines that are only about one degree less corny than Hummel. (The children aren't quite as chubby-cheeked.) I can't remember what I wrote, but the reason I was asked to do it was because Lladro was an advertiser!!!
That's about the most unethical thing I ever witnessed in my 45 years in journalism. It happened a few times at the New Orleans Times Picayune (where the Lladro assignment happened) in situations I wasn't involved in but heard about. My first newspaper, in a small town in Minnesota, had a special edition once a year called the "Progress Edition," which included stories about local businesses (i.e., big advertisers). They at least paid us extra to write them, but imagine having to write a long story about a grocery store! (The bigger the company's ad budget, the longer the story, and needless to say the story had to be flattering.) At the Duluth News-Tribune, a freelancer once wrote about how you can sell your own house and avoid paying agents' fees. The local agents were so outraged that the editor wrote a ridiculous apology column.
On the other hand, I never saw even the slightest hint of what many people assume goes on in mainstream media outlets all the time: stories written to advance the owners' political agenda. No editor ever gave such instructions to me, and I never heard anybody else mention having it happen to them -- not even over beers!