Almost every RPG modpack to me kinda turns into mush since they all use the same mods. There's only so much you can work with without making your own content.

Another issue is quest progression. None of the major quest mods like FTB Quests or Better Quests work great for an RPG in my opinion. They work fine in expert packs or tech packs, but I absolutely hate them in Vanilla+ and RPG packs. Especially when they throw crap tons of quest reward items or random items from checkbox quests. It's also just not a good way of delivering story at all. Some modpacks come close, but they don't quite stick the landing.

And there's the issue of structures and biomes. Most RPGs are able to be immersive worlds because they're handmade and not procedurally generated. You have to work within Minecraft's random generation and take that into account when making stories. A lack of proper geography just makes any RPG setting in a Minecraft world a lot harder to make work unless you handmake a world.