You know, I think I've figured out what makes the novella/audio drama so off-putting to me, apart from the obvious material.
You see, no matter how serious the situation was, everything in FFX(-2) was romanticised: You had these beautifully-coloured, varying environments and characters; music was constantly playing in the background and you had these cinematic, sometimes exaggerated camera angles. It takes the word 'fantasy' quite seriously. Because of all these factors, even devastating scenes like Kilika and Home's destruction or the failure of Operation Mi'ihen seemed almost poetic or mournful, rather than instilling terror into the player. There was also the pacing of the dialogue, and even in the Japanese version most of the dialogue was written in a poetic format.
But with the audio drama and the novel in particular, everything seems a lot more raw. They seem to get straight to the point with everything with no priority over subtlety when it comes to topics like death. Because we have no visual or audible stimuli, nothing's there to really 'distract' us. We have everything straight-up told to us through dialogue and narration, so graphic descriptions like Chuami's mother's death and Tidus' decapitation become a lot more shocking. If, say, we were told about Home's destruction in written format only, the revelation would be a lot more surprising.
Whether or not this is a good thing is entirely up to opinion, and in my opinion it doesn't really click with me. The FFX saga is an inspiration to me because of its world and its 'show, don't tell' mentality for most things. I think that this is simply just a case of using simpler mediums, since obviously SE didn't want to make a half-hour long cutscene exclusively for a remaster.