I'm not sure where I stand on it really. I don't mind Union X as a whole. Mobile gaming is popular and KH dives into all sorts of gaming genres so in that regard, why wouldn't it make a mobile game? I'm not into mobile games but I know many others are and since a lot of people play these games, there's no harm in that right? So gaming wise, no problem. Not everyone is into mobile gaming, but for those that are and enjoy playing it that's great for all involved.
As for it be connected to the story, hmmm. I did like the Back Cover movie they did for Chi, was it? I know it didn't cover everything but it did enough to get me interested in the Zoo-Keys (Foretellers) so maybe once Union X gets a movie I'll be more invested in it personally.
My overall problem with it connecting to the main story comes to two things. One of them is something others have said. Having these old characters being in this timeline. Currently we have Marluxia, Larxene, and Ventus in the Fairy Tale age and the bulk of this Darkness story so far revolves around Laurium and Ventus. Two characters already established in canon. Yet we have new characters like Ephemer, Skuld, and Brain who don't feel fleshed out at all despite being around for a pretty long time now. They feel like a brand rather then characters that can stand on their own, and I wish this Darkness story was more about them then Ventus, which honestly feels like a retread of the Vanitas story. I've already nitpicked Marluxia and Larxene in great depth on why I'm not really a fan of what they are doing with them, because they are basically new characters in old character skins. All of them have inconsistent narrative now, and it feels rather silly how these characters are this old and never get memory recalls of events that happened to them, despite being around similar situations that should trigger something. Why does Marluxia's second death start to trigger his lost memories but not his first one? I am going to keep a open mind about this, but needless to say I would prefer it if Union X didn't have all these old characters in it.
My second concern with the story is it feels to samey to what we just did with BBS. A mastermind from the past before the events of KH1 is coming back, aka Master Xehanort. He existed with Braig/Xigbar and some Organization members before they became Nobodies, Ventus is here with a dark side called Vanitas. And now we have time displaced characters who didn't age fast forwarded to the future. It looks like we could be setting up the same thing again. Mastermind from even further back into the past (Master of Masters) before the events of all KH games is coming back. He existed with Luxu/Xigbar and some Organization members before they became Nobodies, Ventus is here with a dark side called Darkness. And potentially more time displaced characters are coming to the present, already teased with Subject X. Does this sound familiar? I know the series repeats themes, but this is almost copy and paste. But at the same time I'm at least excited for Master of Masters and the Foretellers, not so much the Dandelions. And Darkness would probably interest me a lot more if it wasn't tied to Ventus. I like Ventus, but if you are going to use him do something new with him instead of all this darkness pent up in him story.
So overall I don't know how I feel about it. I like the idea of telling these old stories from before the age of Xehanort, for lore purposes and history, like some of the Star Wars lore stories. Maybe even dropping some Easter eggs like how most Final Fantasy games references it's other games, but never goes deeper beyond. But the way it has basically forced it's way into current KH narrative to the point it hijacked a chunk of KH3, and making it known the next saga is more digging into the past, rather then setting up the present and future has me unsure. I could go on, but this is long enough. So I guess I'm half and half overall.