I don't disagree with much of what you guys say. I truly admire the balls behind Derren Brown's Ghost Train as a project; the concept is frankly nuts, and spending that much money on something so unique and ground-breaking was a very daring move on Merlin's part. Some of the components it entails are incredible.
However, I don't think Merlin can necessarily be blamed for its poor reliability. The ride is/was highly experimental, and using very sensitive technology like VR, so it was always likely to have a rocky journey in terms of reliability.
Also, I'd argue that even had it had perfect reliability, there were inherent flaws with the idea in hindsight that would always have limited its success.
For starters, I've heard many who rode it say that it had a bit of an identity crisis. A common theme I remember from early reviews was that that Derren Brown's Ghost Train had many elements to it that were really good in isolation, but didn't work when they were blended with all the other elements of the ride. The experience was apparently a bit incoherent, which made it feel a bit weak overall in the eyes of many. I also heard a lack of consistent quality throughout all sections mentioned, with things like the actor section and the second VR section being commonly perceived as vastly weaker than things like the first VR section.
From a wider "success" standpoint, one thing I would say about Derren Brown's Ghost Train that I've never heard anyone mention is that in my opinion, its appeal was possibly too niche for a major headline attraction. An alleged £30m was spent on Derren Brown's Ghost Train; at very least, it was referred to as "Thorpe Park's most expensive attraction ever", so its cost beat the £18-20m spent on The Swarm. When you spend that much on a major headline attraction, you want it to have fairly wide appeal. Something like a big thrill coaster has fairly wide appeal; even a big coaster with a 1.4m height restriction arguably appeals to the entire thrill-seeker market, encompassing older families, teenagers and adults. A more traditional dark ride also has fairly wide appeal. Does a 13+, actor-led, horror-themed VR dark ride have wide appeal, though? I'd argue that it doesn't; that type of thing only appeals to a very limited cross-section of the thrill market that likes horror, scare attractions and such. Had DBGT been a filler attraction costing only a couple of million (think along the lines of Sub-Terra), this wouldn't have been so much of a problem, but ultimately, it was very big and expensive for something with such niche appeal in the grand scheme of things. I would argue that placing all of their eggs in the basket of things with overly niche appeal is one of the biggest wider problems that Thorpe Park has suffered from for the best part of the last decade, but Derren Brown's Ghost Train was probably the most disproportionately expensive/major attraction that the park has built relative to its appeal; all of the park's other investments of this ilk were at least fairly low-cost, so it didn't matter quite so much.