On the recommendation of several people on this site, I signed up for two Mercat Tours in Edinburgh.
The first one I took was Hidden and Haunted. The tour started with a description of the execution of Walter Stewart, Earl of Atholl. This is probably the most gruesome story you'll ever hear in your life. (You can get a quick overview on Wikipedia: https://en.wikipedia.org/wiki/Walter_Stewart,_Earl_of_Atholl). The version our guide told went into far more horrific detail. One woman on the tour actually got so sickened that she had to leave the tour. But I stuck it out. After that we went down into the underground vaults created when a bridge was built. There's really nothing to see down there, and the stories she told were about ghosts that supposedly have been encountered by various people in the vaults. I didn't think this tour was interesting or fun or worthwhile.
The second tour I signed up for was Secrets of Edinburgh's Royal Mile. I included the optional "Skip the Line" visit to Edinburgh Castle, with a 45-minute introduction to the castle. It says on their website, "you’ll walk straight over the drawbridge, and straight past the ticket line."
All the participants were waiting at the Mercat Cross for the tour to begin, and there was no guide. Finally, 5 minutes before the tour started, a representative from Mercat showed up to tell us the guide was sick and the tour was canceled. We were given the option to take sign up for another tour that afternoon or another day, or to get a refund. I asked whether we could still get our entries for the castle; he said no. I asked him how it was that Mercat, a large company that offers many different tours all day long, doesn't have a backup guide available when one of their guides can't offer their service. He said he would pass that suggestion along.
Fortunately I was able to get a ticket to the castle that afternoon at the TI. I'm not sure others were so lucky. And there is no line to be skipped, because the castle was fully booked, so the ticket office wasn't even open.
I don't recommend Mercat Tours based on my experience.