My experience with Tripadvisor and Viator.
We do a lot of minivan day tours when we visit Europe as we prefer not to deal with a car. If a location is easy to visit by train or bus we'll usually go on our own because we like the flexibility. But there are often things worth visiting or the ability to see several things in a day that are more easily done with a driver.
So - TripAdvisor is a good resource for evaluating local tour companies and guides. They seem to cover the gamut - between single person tour guides and large local companies. One of the things I usually end up doing is try to figure out which local tour company is being used by a consolidator like Viator if there is a particular tour of interest (there are several consolidators). They work hard to hide this information!
Researching Barcelona, it took quite a while. Finally I happened to stumble across a tour description that mentioned an approximate meeting location, and Bingo! on google maps I could see that Explore Catalunya was nearby, and sure enough their tour descriptions matched. Then I was able to read reviews specific to that local operator. We ended up choosing a different company because they had options that better matched our goals. I think I found that one mentioned as a very reputable local company in a conversation on TA.
Once I find the local tour companies through various searches, I study the reviews on TA. This can be very helpful - you quickly figure out if someone is unreliable or sold a business, etc. For example, there was a tour company out of Avignon with some great looking trips, several of which interested us. They had a nice web site, detailed descriptions, etc., but until I read the TA reviews, I had no idea that the business had likely been sold about a year prior, and reviews had mostly disappeared - just a few disgruntled ones from a few months ago. There are plenty of bad reviews on TA. So it's not like someone is deleting them.
BTW - I don't think there is a problem with using Viator, and as far as I could tell for the minivan tours they seemed to be about the same price as going direct. I just prefer to deal with the local tour company directly if possible.