I can say that we have had reviews done in the past that we submitted to vendors for comment, and we told that we would lose advertising if we did not remove some of the harsh comments.
Ouch - how unethical - obviously if you are living off advertising that can be difficult to deal with - I would have thought that offering them the chance to have included in the article their response and details of how they intend to address any identified shortcomings would have been the ethical way forward. It also opens another can of worms - should you be accepting adverts for products you know to be shoddy? A bit of an extreme analogy but should a financial site show adverts from load sharks - I think not - so where do you draw the line.
Look at all the software you use, and then how many reviews there are. There aren't a lot.
I do agree that the more "enterprise" the application the less available the comparative reviews are - I think this is probably due to a mixture of:
a. The audience for the reviews is much lower so it would not be as compelling to devote time to writing an article
b. The software is much harder to review anyway
c. The software is expensive so if it is not provided free for review it would be difficult to justify purchase for review
d. The infrastructure required for and installation of these products can be quite major
That said there are consultancies (Bloor, Gartner etc) that make money from reports on this sort of thing.