You're speaking of fun (subjective), Xenosys is speaking about complexity/intricacy and difficulty overload.
I do not have fun in ultimates either because like you I care more about job intricacy than encounter mechanics or even difficulty (to a point).
The hardest fights used to be the puzzle mechanic fights in savage usually, so the last fight of each tier. You can compare them to current 4S p2, 8S p2, 12S p2, etc. Those aren't the hardest anymore by far. Mechanically, things like Superchain, Gorgons, or even Pinax etc (aka doorbosses), can feel a lot more involved, fast paced and very stringent on mobility (which is where big hitboxes and less cast times are probably meant to help... rip RDM/BLM). More generally, I feel that the difficulty has increased tenfold for the earlier fights of tiers rather than the last fights. P5S was noticeable already but P9S has felt especially gruesome to prog for a first fight and required for most of the people I talked to, including myself, almost twice or thrice the time then older equivalent fights like O5S, O9S, etc... Even some extreme trials have increased in difficulty, like Golbez notably. Even P1S, while easy, was a lot harder than the very first savage fights of older expansions like A1S, O1S (lol), or even E1S, just because it had an actual puzzle mechanic (a puzzle in a first fight??? even if easier, that's literally never heard of so far). Now take a look at P10S and tell me if that wasn't gruesome for a 2nd fight in a tier?
I'm not saying it's systematic and noticeable everywhere, and difficulty is a very subjective thing, but it directly ties to my own experience. I am a semi hardcore raider that has raided since Stormblood, and the more expansions released and the more I felt that it become actually difficult for me to follow and learn. Maybe I'm getting old? But more seriously, especially Abyssos and Anabaseios have proven excruciatingly painful for me when it came to my most hated mechanic so far that is Superchain (I'm still unable to properly trudge through 2A and 2B without callouts and delaying burst). It's been especially frustrating and depressing because jobs and especially the battle system is supposed to be easier right? Yes, but no. It took me long enough to actually admit that yes, fights are getting harder, are shifting in paradigm and asking for a lot more precise DDR paying attention to countless things at the same time on the platform and the encounter compared to previously: you used to pay a lot more attention to your rotation (and various things like enmity for tanks, MP for casters or rPhys, etc), while nowadays a lot of your attention is required on the arena itself. My point is that, I enjoy RPG mechanics and player agency and rotational choice much more than I value encounter mechanics. Incidentally, it also makes me very good at the former and pretty average to sometimes passable at the latter.
Needless to say, I hate that shift.
Note on Ultimates: I have progged UwU halfway, TEA past phase 2, and DSR up to time reset. This has been enough for me to say that those fights, while definitely very difficult mechanically on top of additional things to deal with like short mini bursts and multi target scenari, just do not respect my time. You eventually end up having to go through 10+ minutes of fight again and again and again just to have the luxury of maybe see 10 more seconds of prog where you're currently stuck at. I feel this is artificially inflated difficulty at best, and I do not find this enjoyable, but a lot of people seem to, so props to them.
This also indirectly ties in with what I said above: while it's true for high end challenging content, it's not for casual content (roulettes, storymodes, dungeons etc). Difficulty has remained at a baseline level and hasnt much changed over expansions. If anything, the EW Alliance Raids have been a boring cakewalk comparatively to the more involved Mhach, Ivalice and Nier. Meanwhile, there is little left of the battle system and jobs have at least for some of them been very dumbed down, and/or dullified.
It seems to feed into a toxic logic of player castes where casuals stay in casual content, midcore players in extreme trials at best, and hardcore in savage and potentially ultimate.
YoshiP said it himself: go play ultimate if you're not happy with easier difficulty segments..........