I don't think you can blame the increase complexity of the game for the poor storytelling. PS1 game were a lot more complex than SNES game which were a lot more complex than NES game, yet the storytelling only became richer over time (or at least didn't become worse). I'd argue jumping from SNES era (2D) to PS1 era (3D) is a much bigger jump in game complexity than PS1 to even PS4.
As far as specific:
12: It had some cool idea, but it was just too spaced out and not focused enough. I remember partway trough the game you have to get a sword to cut crystal, but no one ever tell you why you even need it (and it turns out you don't). Then later on some ghost things shows up out of nowhere and are never really explained. What happen to the story of plucky rebel retaking their country from invader? Plus most of the cast had literally no reason to be in game, shamefully including the main character (later on there's some scene where important plot element are being discussed and he's literally in the background playing with rocks or something). The setting is also woefully underused, it's world with plenty of non human humanoid yet 5/6 of the party are human.
13: Jesus Christ WTF happened there? I could go on and on, but whose bright idea was it to have the main character literally do exactly what the bad guys want, including at the very end of the game? The bad guys of FF13 literally win because the main character are too stupid to understand that when god Hitler tell you to press the "kill everyone" button, you shouldn't do that. Also most of the game is just wandering around trough random area that have no logical connection to each others and don't have any story purpose anyway. Oh and you have to read a light novel to understand it (both figuratively in the sense that you need to read a bunch of data entry and literally in the sense that there's a light novel with key plot point in it).
15: Having learn nothing from FF13, they decided to double down on the "package the story separately idea" and you get no character development in the game. This leaves some baffling moment, like when the main character love interest is the in the game literally less than 5 minutes, making her eventual death un impactfull (I had more attachment to the random guy who work at the dinner next to Cid garage since I at least interacted with him a couple of time). To compound this problem, her death serves practically no purpose (you only see leviathan once later on and any other summon would have been just has useful) and it also ended up destroying an entire town, probably killing thousands of peoples and making the rest of them homeless. Making her a bigger villain than the actual bad guys. The best part of the game is without a doubt the road trip aspect, but this parts has nothing to do with the actual story and once the story starts for real, the road trips aspect disappear with it. Oh and along with it all the main character become whinny emo downer, so you know, good bye fun interaction between them, welcome blame shifting.
But that is also what I think was the problem with FF15 and espescially 13; that it lingered simply too long in the planning phase and had to cut too many corners to introduce the characters, background lore and story. There is tremendous ambition but often it just doesn't really work out.
