Although it's never been my day job, I've written for magazines for about 18 years, and flying magazines for about half of that - I enjoy it and the extra beer money is nice.
The point about advertising is well made, but it need not interfere with editorial independence too much. I've had editors make it clear that they wanted me to write about subject X because it would sell advertising, or that if I'm going to criticise product Y I'd better show the editor in advance my clear reasoning for when I got the irate phone call he could defend it - both of those are fair enough. I've even lost a monthly column (after about 4 years) in a flying magazine because they were rebranding and the type of flying I tended to write about wasn't in keeping with the new intended market - irritating but fair enough.
There have been cases where writers have been sacked or their work refused because it criticised something by a major advertiser; I'm not going to name them, but those in the business know where it's happened. That is unacceptable (to the readers and writers anyhow) but thankfully very rare and does tend to find itself out. So, I don't think it is, or need be, a major problem with the integrity of aviation journalism.
I did write a flight test for one of the big 3 UK magazines a while ago where I was quite critical of several aspects of the aeroplane. The editor asked that I confirmed I really meant what I said and could prove it - then happily carried the article. About a fortnight after it came out, I got a private Email from the aircraft company, very polite saying "you are quite right, and here's what we're going to do about it".
So, not all is bleak.
G