I am anything but a casual (in my opinion, at least), and I found it unforgiving regardless because it's not a well-implemented mechanic.
Kingdom management is much too random and in the late game can become impassable not because you're bad at it but because your highest stats have success chance of 30% or so against the checks it throws at you. I've beaten the game once on Challenging and the part where you are bombarded with ridiculous unfixable hits to your kingdom stats (Pitax) that can ruin your game unless you safe-scum a lot or just switch it to cheat mode is NOT well thought-out. Neither are the late-game fights where you are drowning in enemies that have godlike stats and a lot of DR/resistances. The Bald Hilltop, on the other hand, was not a problem at all. It was more of a nuisance and a really weak plot anchor rather than an actual challenge.
I used to stand up for it back when it was being bashed for being unplayable on release due to how difficult it was at the start (The Old Sycamore and such). Having finally played it to the end after two enhanced editions and a myriad of patches, I, in retrospect, don't really agree with my assessment back then. It's a design disaster for the most part, and unless they intended for the players to min-max to hell and back, I see no way to explain why the game has an abundance of enemies with ridiculously overtuned abilities (+8 to AC from Dexterity on a random cat is insane) and that can hit you so reliably that any AC value below 30 in the early-to-mid game is already too little for your frontliner. It's not interestingly challenging, it's a stat bloat through and through. And people complain about how BG3 does that, when that one troll in Kingmaker's penultimate chapter was almost unhittable by my divine hunter PC with a +5 weapon, 26 Dex, and a bunch of active spells without casting True Strike on top.
All the class choices and multiclassing possibilities are nice, sure, but how many of them are actually functional against what the game throws at you and how many are just plain garbage? The alchemist is pretty much mandatory to have, as spamming difficult fights with force bombs saves you a lot of trouble (tactics, planning? Grenade spamming!), while someone like the crusader can neither do their cleric duties right nor hold their own in melee and ends up hampered in both ways. Something like the magus is a fantastic concept that is fun to play and build - but they melt in melee and can't hit crap without constantly using their weapon enhancement ability because of the lower BAB, so I had to scrap my very first playthrough because I kept dying to bandits and start with a regular ol' fighter (well, Aldori defender) instead.
As for the writing, maybe it is indeed dependent on whether or not you like the setting to enjoy it. I can't really take Golarion seriously, though. It took the edgier/more nonsensical parts of the D&D settings and cranked them up to the max while still clearly just taking a lot of the concepts and using them as their own. The pantheon alone sure is a... collection of extremes. And as far as Kingmaker specifically goes, it really makes you hate their version of the fey with a passion. Will-o'-wisps especially.
About gameplay - I'm so glad you said it because I am pretty casual and in some circles it means I can't have an opinion on the gameplay XD. But yeah I agree, especially when it comes to the fact you have millions of classes, but if you only count viable ones you have much less, and even if you can make a less great class work, you have to be super powergamer minmaxing to do so. About stat bloat I also agree, and that's why I call it buffinder. If that is too much annoying for you to play the game, you're out of luck because wrath of the righteous is the same.
About crusader system and kingdom management - unfortunately I feel like the crusader system is not much better. Fortunately you can easily remove it from your playthrough.
About story and writing - I generally agree with what you're saying here. However, I think the owlcat writers are much much more better than the TT buffinder writers are. That means that most original content in wrath of the righteous is amazing. I really really hope owlcat next game will be original, or at the very least not based on some existing buffinder adventure. At the very least, while these extremes you talk about are still in the game, the overall writing is still a big improvement.