I'd say that brand bias is something that one can't avoid. If you are very happy with the vehicle you own and the service station guys are simply addressing every single issue in the vehicle then you are going to recommend the vehicle to others, even if the vehicle overlooks a requirement or two of the person - but then the things overlooked are compromised by good A.S.S. and product quality.
Its just that if one highlights both pros and cons of their car then they are unbiased and if they highlight only pros then they are biased, hiding the cons or manipulating the cons is something I'd say, fells in the category of misguiding the person who approached you for advise. Well, its better not to advise other person than misguiding them IMO
But yes, I personally believe in keeping away from giving opinion of mine about anything that I have not experienced quite well(that's the basic reason I keep away from Zest threads). Why distribute partial knowledge after all?