As a former agent with @Kingdom Konsultant I can shed a little light on this particular area. I can't speak for every agent or agency, but this was my experience.
I was in no way obligated or ever instructed by Disney or the agency owner to give a positive review of Disney or a negative review of something non-Disney. The agency owner stressed honesty with our clients more than anything. Without it, our recommendations meant nothing.
If I went in full tilt pixie dust spewing fanboy, most people would assume that I was lying or reading from a script.
Now if a client asked me to get them a reservation at the Luau, I would not answer with "Are you crazy? That thing stinks! Don't waste your money!", I would get them the reservation.
However, if they asked me what I thought of it I would give them my sensible review. "The show is enjoyable, but a little too corny for my taste. The food can be pretty polarizing. It tends to be a little overly sweet and it would seem that people either love or hate this. My needle points a little more toward the hate it crowd."
