Commercial magazines serve two complementary ends. They need the support of advertisers in order to publish, but if the readers don't like and trust what they read, the advertisers will wind up with no audience. The best solution to this problem is what author and editor Don Sutherland calls "the separation of church and state." The editorial department doesn't tell advertising how to hustle ads and advertising doesn't tell editorial what to write.
By and large it's worked well for the publications I've written for; I really couldn't be successfully employed under any other policy for very long (PhotoElectronic Imaging stopped using me because I wouldn't write inaccurate fluff.). In 30 years I've only had three articles killed because the publisher was afraid an advertiser would be antagonized.
The major manufacturers are all respectful of this. I've written critically of Kodak, Agfa, Ilford and Beseler products (among others) and not once gotten a bit of grief from any of them. I was told that after one particular "buy but with substantial reservations" review for Beseler, the president and owner of the company held up the article at a meeting and said, "We need more reviews like this! Nobody's going to think this is a puff piece. They're going to believe his recommendations and that will sell units."
When I demonstrated that there was a significant oxidation problem with black-and-white RC prints, especially with Agfa paper, Agfa didn't complain nor try to block publication. Instead they requested as many details for me as possible. As soon as they were convinced of the reality of the problem they halted production on the paper and pulled stock from the warehouse. A very smart move; it saved them from losing a product liability suit several years later. Up until the very end, we had a close and excellent relationship.
Similarly, revered columnists like Herbert Keppler often spoke their critical mind in print and he probably had the tightest working relationship with the photo industry of any U.S. magazine writer. That's the good news.
One former editor of mine absconded with a review Nikon system. Packed up, moved away and said, "I'm keeping the camera and if you want it back come after me." Another editor kept a Contax that he asserted was a gift and Contax asserted was a loan. One fellow columnist was notorious for strong-arming manufacturers into giving him free stuff in order to get coverage; if you didn't give him product, you didn't get mentioned.
In a most egregious case, an editor for one of the major magazines refused to return a high-end Kodak DSLR and when pressed claimed it had been "lost." Kodak was pretty damn sure it hadn't been. I said to my source at Kodak that I hoped they had gone after him for pocketing $20,000 worth of camera gear. They said that they hadn't because they were afraid that if they did the magazine might retaliate with bad press!
In fact, in none of the aforementioned situations did the manufacturers do anything except gulp and swallow their losses. I call it the "balance of terror." While magazines may seem pathologically afraid that manufacturers will pull their ads if they're given bad reviews, manufacturers are equally terrified that they'll get bad reviews if they complain about anything. Despite the extreme rarity of such occurrences, both sides are insanely gun shy. Ridiculous!
Only a few times in my 30 year career has a manufacturer retaliated (or threatened to) over something I wrote. Fred Picker and Zone VI refused to pay their bills to Camera & Darkroom because they didn't like some factual (not even unflattering) press they got. Berkey Marketing pulled their ads for six months after I unfavorably (but accurately) reviewed a darkroom timer; my editor and publisher stood behind me the whole time. HP Marketing threatened to pull ads from C&D if we didn't do them some unreasonable favors. Editorial didn't cave in and they didn't pull their ads, but they did slander me online for years afterwards.
Personally, my way of dealing with this kind of extortion is to eliminate their power by cutting off their access and their PR. I blacklisted all three of those companies; Picker and Berkey are long gone, but HP Marketing's still around (as is the guy who tried to strong-arm us) so my ban on them is still in effect. That covers a lot of brand names, but there's no shortage of products for me to review from companies I respect.
Yeah, it's a brutal business, but then I knew this was a silly job when I took it.