I have just returned from Edinburgh. Not a first visit to the city or to Scotland. In the past, I have had a car, but this time was car-less. Only you can decide what you want out of a day tour, but I can relate my experience with two different companies.
Heart of Scotland Tours - I booked a trip to Alnwick Castle and Lindisfarne (Northumbria - not Scotland, but, interestingly, Edinburgh was the only jump-off spot I could find offering day trips to those locations). About 3 days before departure, I got an email and text that they were canceling due to low numbers. I was offered a refund or transfer to their "Best of Scotland" tour which was running on my date. After some text back and forth, Heart of Scotland suggested I contact Timberbush - a competitor company - that did seem to have the tour and date I wanted. I thought a lot of this courtesy and professionalism - really appreciated it. As a result, I transferred my booking to their "Best of Scotland" tour. Frankly, I was not interested in the stops, but wanted to support the company for their support after canceling my booked trip. I thought maybe I would be pleasantly surprised. Instead, I found the tour a bit boring - I should have trusted my instinctive "not interested". However, I did think the guide was fine and timings and stops well organized. Still happy to support the company.
Timberbush - I used them for the Alnwick Castle and Lindisfarne tour. Before booking, I contacted them - email - and asked if this tour would actually be running on the dates I had available. Immediate responses with several suggestions. I appreciated their professionalism also. This was the tour I wanted - enjoyed it. I always prefer DIY and would have liked more time, but an organized tour is what it is. Given that DIY was not an option, the tour was quite acceptable for me. Again, guide was fine as was the driving and overall tour organization.
I get the impression that there are a number of good day tour companies out of Edinburgh. Based on my experience (I was a solo traveler), I would expect that some individual tours may be canceled (true any where in the world in my experience). If a particular tour itinerary or date was important to me, I would contact the company and ask about the expectations for go or no go. I was impressed with the honest answers I received from both of the companies above - certainly I didn't sense a "bait and switch".
I agree that it's hard to choose a day tour on a first time visit. All I can suggest is a careful reading of the stops, some additional website research on the sites, and then go with that instinctive sense of yourself!
adding based on above - I wanted to try a Rabbie's day tour as they seem to get high marks from most. Rabbie's didn't offer anything I wanted. (I have seen many of their stops in past trips.) Based on my experience with a tour itinerary that didn't resonate with me despite a fine company, not booking Rabbie's just for the name was a good decision for me - but not necessarily others who like their offerings.
