Different people are able to suspend their disbelief more readily or less readily, and in different ways. The story within a story in TPB isn't inherently less capable of engaging the viewer just because it's overtly a story being told to a kid - it just depends on the viewer.
That is not untrue, but a story within a story just makes it much more difficult. You need more skill to pull of a story within a story instead of just telling the story itsself. And it needs to add something to the film/book/game.
Now, I don't have the biggest confidence in the writers. They have like 7 movies as writers under their belt, on IMDB they are all in the rating range of 5-6,9 and only one is above 7 (spiederman homecomeing, which has a total of 8 writer attachted to it).
So these guys write medicore scripts at best.
Why is a story in a story harder to write? Because first of all, you need to make the viewers suspend their disbelief twice. First for the frame-story (the D&D game group, that was proposed here) and then for the In-Game-Story again. You just now doubled your workload to make the people forget for two hours that they are watching a movie.
Second, it is broadly associated with two kinds of movies: Childrens Movies (a lot of christmas movies do that, because it is supposed to remind the children of the grandma/pa reading them a story in front of the fire place) and true stories (this is the true story, told by me, played by this famous actor)/Fake True Stories (Titanic). --> Because D&D will be a fantasy movie, a frame story (proposed as a D&D Game) can lead to a reception and impression, that this will be a kids movie and it will be harder to reach an adult audiance.
Third, which is connected to one: it decreases any kind of urgency and stakes in the Ingame-Story. It is way harder to suspend your disbelief, that this story is real, when you are told, that this story just takes place in the heads of the characters of the frame-story. And even worse, when ever the frame story is mentioned, the supension of disbelief is destroyed again and needs to be build up again. Thats why it is often used in Kids Stories - because kids can do that way easier than adults. Thats also why it is usally used in lighter stories.
For example, there is the Movie Sucker Punch, that tries to do that. The Frame Story is the one about a girl who is put into a mental asylum, that will be lobotimized in 5 days. The inner Story is about her and her friends fighting a way out of the asylum. And it just doesn't work well (6.0 on IMDB, 22%/46% on rotten tomatoes - at best medicore). Because, even so they try to make the fights in the inner story matter for the outside story, it is shot in a way, that you now that these fights are not real. And this unrealness of the inner story is strengthend by the frame-story.
If you compare that to Scott pilgrem vs. the world, which has a similiar structure for the story (magical fights to achieve goal x) - it works way better (7,5 on IMDB, Rotten Tomatos 82%/83%). And one of the reasons is, that it doesn't have a frame story. If scott pilgrem would be told by and old scott, who tells his children, how he met their mother, or if it would be about a scott, wo is in a mental asylum fantasizing about getting a girl (proably the reason he is in the asylum), it would be worse (or for the second Idea a Horror Story), because now it will be harder to suspend your disbelief for the inner story (and that is why How I met your mother is a comedy story, because Comedy stories work well with an unreliable narrator).
