I have a great deal of respect for the men and women who write the role-playing games we enjoy. From the outside looking in, it seems almost like a dream job; these people are paid to create something that thousands, maybe millions, will enjoy. Depending on how well their work is received, it's content could persevere for generations to come and even change the way genre narratives are delivered. That said, I can only assume a great deal of tribulation goes into their process. Characters have to be created, worlds have to be defined, themes have to be set, and conflict has to take place. Linear RPGs are not the dominant force they once were, and it's become hard to get away with the same tropes and character arcs. Complexity is built into RPGs with the understanding that that RPGamers may not take the game seriously otherwise. This can lead to really interesting experiences, but I'd argue that more and more genre writers are making the experience too complex — to the point of hampering the story itself.
Effective storytelling is challenging in any medium, but especially in an interactive media format like video games. RPG writers have to find a way to balance character development, insightful dialogue, and exposition with gameplay. This process is sometimes made easier in western RPGs, where your character is usually a blank-slate everyman who needs to become acquainted with the world to survive; however, more traditional JRPGs often don't have that luxury. Ni no Kuni employs the Wizard's Companion book to detail almost every element of the game's world, with the expectation that you continue to reference it as you adventure. Similarly, in Final Fantasy XIII we're are given a set of distinct personalities amidst a plot that is already in motion. A bombardment of unknown variables is then detailed by an in-game glossary known as a "Datalog." This mechanic becomes useful in understanding the universe, but should it be there is the first place? Exposition should be carefully woven into the plot, and a reference book isn't effective narrative delivery.
Ignoring the existence of Final Fantasy XIII-2 and the forthcoming Lighting Returns, I'd argue that Final Fantasy XIII features an interesting world full of well fleshed-out lore. I'd also say that you likely wouldn't recognize that unless you had fully explored Gran Pulse, found and deconstructed the meaning behind every analect, reviewed every entry in your Datalog, and replayed the game a couple times. To that effect, my problem with Final Fantasy XIII's story does not lie with setting, mythology, characters, themes, or dialogue. The game's unintuitive narrative delivery and the abuse of unique terminology, two elements which many modern RPGs contend with, ultimately hamper the experience.
Unique terminology, proper nouns in particular, can be disorienting to any gamer who isn't well versed in a particular game's world. Being used to more standard words like soldiers, crystals, and emperors, question marks often arise when we see things like l'Cie, fonons, and -lambs-. The intent of this terminology is twofold: (1) the writers wish to convey that this world and its inhabitants differ from yours, and (2) by using words that you don't know they hope to hook you into this unique world and — by extension — the overarching narrative. Games like Xenogears and Mass Effect do this well by teasing the meanings of critical terms like Reapers and -lambs- before a narrative reveal, but games like Tales of the Abyss and Final Fantasy XIII meander on important terms like Seventh Fonon, Light of the Sacred Flame, The Score, l'Cie, and Fal'cie — referencing these terms frequently, long before an explanation as to what they mean is provided. In that sense, RPG writers also have to walk a fine line in their use of world-centric terms. A term that is used too frequently, without adequate detail, usually serves more as a roadblock that takes the gamer out of the experience.
It would appear that complicated narratives are widely accepted and appreciated by RPGamers, up to the point of player head scratching. It's rare that any gamer goes into a game for the first time knowing all the answers. We need mysteries. We have to have questions. Otherwise, what is the point of playing? However, the way that mysteries are revealed and questions answered in-game shouldn't require more work than being present for dialogue and plot events. If a game's narrative is so complicated that a book's worth of information has to be provided and requires several hours to understand all of the proper nouns, it's either too complicated or you're delivering it wrong.