That's interesting, thanks. I'm surprised to see such great feedback for my course and alma mater.
No, I don't think 'any' of the sources of information are objective. NSS is professionally run, but subject to sampling issues, bias etc. That is not its fault, creating an accurate representation of qualitative data is a postgraduate research subject, notoriously difficult.
RateMyProfessor was just an example - a lot of it is silly, but there were also some gems. What I was looking for was detailed explanations of what went wrong and why, which I could then ask current students. I didn't go via any 'official' channels I looked people up on LinkedIn, the local uni Facebook groups, etc. I found people that I had things in common with and ask them, they were very happy to help.
I don't think EVERYONE needs to do this of course. It depends on how fussy you are, and your mileage may vary. As an undergraduate all I wanted was the most prestigious, went to LSE, job done. But I have always been very academic, competitive and career driven.
As a postgraduate I have got very detailed, module level questions, and a lot more to consider so I had to be more detailed.
Of course, if a course is 'small and niche' I'd imagine you don't have much choice to begin with - so it hinges on a few differentiating factors? They don't have ambassadors for every course, so you could ask them to point you to a few of their mates. hang out in the student bar - see if anybody there can. There's a surprisingly low degree of separation, even in a massive university like LSE someone always knows someone who knows someone.