The combat systems in FF by themselves are pretty basic even though ATB adds a cool real-time element. FFX's CTB however I consider one of my favourite combat systems, solely based on the fact that the attack order is visible and can be altered, adding another strategic element to battles.
While it's true that One More is mechanically slightly more sophisticated than many JRPG combat systems, the battles on Normal difficulty can get really, really tedious if you're underleveled, even if you take advantage of weaknesses and use buffs and debuffs. That's at least my experience with Persona 4 and never happened with any of the FF's I've played.
I like FFX's CTB, and wish they expanded on it. As it stands, in random battles it doesn't matter at all, it's just a matter of attacking things with the right person to end the battle in 1 turn, and if you screw that up, you'll survive several turns of beatings. In boss battles, it matters but most boss battles(not all) do boil down to healing and attacking, with a few gimmick boss battles, none of which are difficult if you just don't avoid battles on your travels(no need to go out of your way to grind). The Dark Aeons are the only thing that can be said to be a difficult boss in that game. I'm not saying that FF battle systems are below-average to bad, I'm saying that they're decent and Persona 4's battle system is a notch above. I don't remember ever actually having to use the defend mechanic in an FF, for example.
In either case, I get the impression that most people who played through Persona 4 didn't do so on the highest difficulty. It's not just a health and attack boost on enemies, and an exp and money debuff on you; the random enemies actually lose some elemental weaknesses and gain elemental strengths. There are a lot of random battles where the enemies have no weaknesses, and take advantage of yours, which makes the individual random battles more interesting. It becomes a gauntlet run, to see how far you can get on a single bar of SP(Granted, this became marginalized in Golden with the SOS mechanic). How is that different from SMT, you ask? Where in SMT if you're running low on SP you'd go back to an inn(Well, in SMT IV you can just run around in circles to get your SP back with apps), in Persona 3-4, you have to give up a day or pay an absolutely absurd amount of money to the fox/clock. You have a finite number of days, and losing them is the loss of an opportunity. This adds a layer of scheduling and conservation to your dungeon crawling. This is what makes the random dungeons interesting, they're a pure endurance run, with a consequence for tapping out. The dungeons in SMT are also an endurance run, with traps, puzzles and twists and turns(A little less present in SMT IV, but that aside), but there's no consequence for running away and going back to an inn(Well, Inn equivalent anyway).
Stealing, morphing, blue magic learning, using magic etc. may be optional in FF but you make the games much too unnecessarily long if you don't utilize these. You will have to solely rely on money and exp/level from grinding to defeat enemies otherwise. If you want to advance at lower levels (i.e. not spend time grinding beyond regular encounters) you have to make the most of each battle.
Each time you fuse the number of demons you can use in future fusions is reduced. You need to bring in new blood or you will run out of demons to use.
Also, fusing two very strong demons will result in sacrificing useful abilities as the number of ability slots is limited. Even if they both have it and you don't sacrifice the ability itself, you will now only have one demon instead of two who can use it.
And negotiation is much more consistent than you make it sound. It randomly may fail even if you choose the options that usually work with the same demon but there are still patterns you can discover and learn.
First: Obviously you need to get more demons to keep fusing them. However, with the smartphone in SMT 4 you can have so many ability slots that fusing two powerful demons might very well be worth it, and you can always go back to the demon compendium. There may be some logic to the demon negotiation system, but frankly it's the worst part of the game for me, worse than people feel about Persona 3-4's dungeons to me.
From an in-game perspective: You're just stopping mid-battle to talk to a demon, for no good reason. After responding to some absolutely random questions from the demon(And even if you answer properly, you may fail), the demon may give you something, join your stock, or slap you in the face. Tell me, why the hell does that mechanic need to be there? The demon auction in Devil Survivor seems like a great alternative.
Second: As to your comments on FF, name me one FF where you can't just spam attack through the majority of random battles and win. I mean, there are other mechanics available, but for random battles you don't need them, and the majority of bosses in I-X(Full disclosure, haven't played III-V) are just a matter of attacking and healing till its dead, regardless of how much you grind, the ones that don't fit this mold usually have some sort of gimmck. Once again, not saying that the battle systems in these ffs are bad, but more specifically, I'm saying that it requires less thought than Persona 4, where any random battle can knock you on your ass in 1 turn.