how difficult it is, is entirely subjective, what you can do to gauge some sort of metric is look at the difference between the amount of guilds that cleared aq40 and the current state of progress in naxx.
on my server https://classic.warcraftlogs.com/ser...ss&partition=3
84 guilds (some pugs) have logged clears of aq40
meanwhile 6 weeks later only 17 guilds have cleared naxx and 14 of them are at 4h.
https://classic.warcraftlogs.com/ser...ss&partition=5
it took my guild 4 weeks to do saph and kel, mainly because it was taking both raids (we do 2x 40s a week sunday and tuesday the day before eu reset) to reach saph and we would have no buffs and very few attempts before the weeks raiding was done (reset). no one really had much FR gear that first week I think, you wouldn't have been able to get that many frozen runes either. (not while focusing on progress)
last week we cleared it in 4hrs with DMF and this week (yesterday) it was done 30 minutes quicker and in a single raid without dmf. as you get more gear, as ppl get used to the fights it gets easier again. the first week was the hardest week. because you only had bwl/aq40 gear. each week your raid has more and more t3 so now (if you have already cleared it) its just being farmed for gear like any other raid. saphiron is a bit of a shitter because you'll likely have to bring more healers if you don't have WBs. some guilds may not have ppl who can switch to geared healer alts and other guilds do.
all in all naxx is a noticeable step up from aq40, gear and buffs make a huge difference, with them the raid is extremely powerful without them its a lot more balanced where you have less room for failure. when your raid is buffed you want to try to get as far as you possibly can with those buffs while losing basically no-one, doing the content isn't so bad but doing it with perfect execution isn't exactly super easy. random ppl will pull aggro and die or die from a lack of heals etc. having it go perfect is something you can only hope for but not guarantee. when a raid is fully buffed, there is a fair amount at stake and an obligation to do your best and carry those buffs as far as you can and get the most out of them.
sure with infinite time anyone can wipe fest through naxx, but not many ppl are managing to pull off good execution.
one example is that with full world buffs ill tank naxx trash with a 2h in berzerker stance and not give a fuck, without world buffs I know ill run the risk of being gibbed one good crit will melt me but with the buffs you have the hp to laugh it off. and your healers ofc have the mana to piss away spam healing through the trash the whole thing is just much faster if you can maintain a buffed raid compared to one that is running no buffs because everyone died or it took too long so they ran out.
killing the bosses and doing the raid isn't impossible, no raid is, doing 10 or so bosses without many ppl dying (preferably no one) is beyond a single persons control but it is the thing you want to ultimately achieve. once its cleared its simply about perfection and doing it well. there is a huge difference between clearing naxx in 1 raid or 4hrs and clearing it in 8hrs or 12hrs. now that we can clear naxx in 1 raid we can go back and chill in aq40/bwl on tuesdays. clearing all relevant content in 2 days. and not wipe festing all week. having it cost a fortune in time and consumes. if we keep this up everyone will have decent gear to take into BC. (the whole point of doing it) maybe we'll even manage a second atiesh.
a lot of the one line posts saying that its easy are probably coming from ppl who aren't doing the instance, its easy if you have a raid spot and are clearing it each week, its not easy if you aren't in a guild raiding it. unlike retail there is no highly puggable version of naxx that you can faceroll with randoms picked out of a hat. you've either been gearing up for naxx since phase 1 or you haven't.
lets also remember that classic isn't as linear or formulaic as the modern game, what stands out to me a lot at this stage is how t2.5 for warriors is probably better for tanking than t3 is on most bosses its the better threat generating set. this is the same throughout the game where key BiS gear doesn't all come from naxx. the best tanking neck in the game drops from c'thun, shaman t1 is better than t2 etc etc etc, this is a consistent thing in classic for various classes and specs. where some of your BiS gear doesn't drop in naxx. the game just isn't as clear cut as it is today. I only use my t3 for patchwerk and 4h where threat isn't a problem and only mitigation matters. classic is weird in this way where things are more ambiguous.
as a tank (but not the MT) each naxx raid costs me 1 titans(i can get away with just 1 for patchwerk MT needs 2), 5x mongoose/giants/defence elixirs (1 every hr), 4 faps (patchwerk trash), 2-3 frost resistance potions(saph/kel), 2-3 shadow protection potions(loatheb/saph), and 2 stoneshield potions(patchwerk). this assumes no wipes. one raid can be done without too many consumables but this is the bare minimum for me. each raid costs something like 250-300g the flask makes up the bulk of this but its still 100g or so in other shit. while you only make maybe 60g back from the boss kills. if your guild isn't as consistent in making progress or you do just wipe over and over it ends up costing a lot, to the point that a lot of ppl probably won't be able to simply keep wiping there.