I think if Heljan gave me a free class 86, I too would say nice things about it knowing they will also be making many more desirable models!
Yes this system of manufacturers giving models for review would lead to some bias I think. It doesn't happen all the time. I'm sure I've seen notes along the lines of "thanks to trains4u for loan of reviewed model" etc
Trouble is the model railway press, and some forums (not this one where free speech is allowed)are manufacturer centric. That maybe alright as long as everyone knows it. Trouble is most are two faced enough to purport themselves as being for the enthusiast, which is clearly not correct. So if your new to the hobby , read the review eg Clayton 17, nip out and buy model on that basis, then find you've got a loco with smoke coming from it, and then worse, find out that it was a known problem- then would you not feel peeved? Similarly with T9s and M7s with limited traction.
I compare this with reviews for cameras, which can be quite scathing or even new cars.