I can sort of get where @Jamesss
is coming from.
Dark rides (any well-themed and -paced amusement ride, really) are a storytelling medium in themselves. You move through the ride at a set pace, with the whole package of sights and sounds carefully timed to play at the right moment as you go by, plus all the little details you only notice in repeat viewings. Sort of like a big 360 degree movie, only there are effects and drops and the moments of anticipation and relief before and afterwards. They can be used to tell a story in a way that's not possible in any other medium. Disney has done this quite masterfully before. Expedition Everest or DLHK's Mystic Manor are great examples.
However, with the involvement of new IPs on a ride, there is a certain risk that the needs of the IP override the coherent narrative. Take for instance The Little Mermaid or Frozen Ever After at Disney World. The need to include certain songs, characters, and scenes from the source material makes the rides play out more like a clip show of the movies they are based on. You jump from one scene to another, possibly with impressive transitions between them, but there's not that same sense of narrative. The story is not told in the ride itself, but in a movie you're supposed to have seen before you ride it. The ride only sums up the highlights. The full experience requires recognition on part of the rider. I'd say it's a poor utilization of the medium, in the same way a highlight reel is not a movie.
A ride built from scratch doesn't have to make concessions to an IP. It can tell a self-contained story, at its own pace, without the need to shoehorn in certain characters, scenes, or songs, and without requiring the audience to be familiar with the background material before they ride. It can tell a story made specifically for the medium.
That being said, IP-based rides can do that too. Tower of Terror is a classic example. Mickey and Minnie's Runaway Railway does this too. Flight of Passage, that Terminator ride at USF, possibly Jurassic Park ... there are many examples of IP-based rides that don't require more than passing familiarity with the IP to deliver the full experience. Splash Mountain itself is possibly one of the best ones out there. You don't even have to know it's based on an IP (which was probably what Disney banked on when the ride was built, but ... well, here we are).
But an IP-based ride can also go the highlight reel route, which would be a downgrade. And it feels like every time an IP-based ride is announced, it's a coin toss whether it will rely heavily on recognition or tell its own story. That's what we mean, I think, when we say we hope the ride isn't based on an IP. It's not the IP itself that makes the ride bad, it's how the IP could potentially overshadow the medium it's told in.