I mean, a great many add-ons are just QoL, from what I've seen.
There's apparently one that lets you keep more than five sets of raid marker presets. This doesn't make it possible to move markers mid-combat (which is no longer possible on the server side regardless), but it does mean you don't have to do the thing some statics do and divvy up who has which markers. (Or deal with the related problem of "Well, the NIN is gone this week; let's go do Endsinger EX instead to get some alt-job Bluefeather weapo- wait, crap, the NIN is the one with the EX3 markers." Not that the EX3 markers PF uses are exactly anything difficult to set up in the right spots, admittedly, but I trust you take the meaning.)
Does it make it easier to clear content? Not really. Does it save time, so you're not spending 5 minutes before the first pull manually setting up the markers that everyone is used to? I'd certainly imagine so.
There's evidently a mod that lets you bind macros to hotkey combinations that aren't used by the game -- or, I guess, that pulls up a radial menu centered on the mouse cursor? -- and some people I have raided with use that for mechanics macros ("Water's safe!" or "T/H Tethers!" or whatnot). Does it give you functionality the game doesn't have? Not really; you can make those macros and bind them on a normal hotbar -- I actually do have mechanics callouts as macros ("/p T/H tethers", "/p DPS Role Call", etc.) for PF, where I can't raid-call over Discord, and have them on a floating "utilities" hotbar where can I click the icon to do a callout.
But does the ability to bind them to hotkeys that FFXIV doesn't use mean you can free up hotbar bindings you'd be using for those? Sure, absolutely. Being able to hit a hotkey for a specific macro would likely be faster than my mousing over the hotbar and clicking the right callout.
This isn't to say there aren't plenty of add-ons out there that do reduce the difficulty of content; I'm absolutely certain there are. Probably a lot more than a few.
But it is to say that not every "QoL" add-on changes the content difficulty; many I've heard of or seen a friend use genuinely are just QoL ones that reduce some aspect of pre-pull annoyance (marker setup for PF), or being able to bind an in-game macro to a hotkey (without sacrificing actual combat hotbar space) you can just tap rather than needing to click the macro icon mid-combat to make a party-chat callout, or whatnot.
Making blanket statements doesn't work for either side of the argument. Pretending that all mods/add-ons are only QoL changes to reduce annoyance is inaccurate -- there's definitely some really questionable stuff floating around out there -- but so is acting like all mods/add-ons change the difficulty in a meaningful way and that none are merely QoL tweaks to the UX. The reality is more nuanced, at least insofar as I've observed it.