Multiclassing (in NWN) sucks unless done exceptionally in order to minmax into a one trick pony (the one trick must be a type of melee).

in PnP you have more viable options but still must be extremely careful not to make a useless character and should never multiclass a caster. (taking PrCs for caster is recommended though, but only if they advance casting)

In NWN1 and 2 the best class is fighter (Champion of Torm is better, paladin and barbarian are nearly identical) in most campaigns, if magic items are very limited then a gish class is better. It is rather boring in theory but in practice a magic user is going to constantly stop for resting and then spend minutes reapplying buffs. Furthermore by focusing on strength you don't constantly run out of carrying capacity which in NWN result in walking so so slowly.

Clerics are excellent gishes and are grossly overpowered. They let you enjoy being a full caster combined with being an good fighter.
Druids, paladins, bards, and rangers are decent gishes as well. Druids power revolves around transformations and IIRC it sucks in NWN1 because magic items stop working when transformed (unlike in PnP where they remain active).

Sneaking just doesn't work in cRPG implementations of DnD because they all incorrectly implement "defeating an encounter" as "killing enemies" and fail to allow for creative uses of sneakiness like putting sleep drought in their wine or setting their tents on fire or some such; if any such situation is thought of it will work for all classes be you a wizard or warrior.
By sneaking by enemies you fail to kill them for XP which means you remain weak and pathetic and in fact cannot even sneak because their spot is higher then your hide and move silently score.

Summoning doesn't work in NWN1 because it applies crippling XP penalties, that also includes a mage's familiar and a druids/rangers animal companion, never use those.
NWN2 fixed that.

Sneaking is actually just fine in NWN, as long as you keep Hide and Move Silently maxed. It gives you plenty sneak attacks with smart use of line of sight mechanics, allows for better positioning and scouting, and turns you into a healbot, if needed. Coupled with Hide in Plain Sight and Rapid Strike, sneaking is completely overpowered, even if kinda boring.

There are also mutiple user—made modules that award xp for sneaking without killing. A Dance With Rogues comes to mind, for exemple. But granted, HotU isnt the most rogue friendly adventure out there. The second act particularly suck.

You are talking mechanics rather then character behavior.
Sneak attacks = DPS vs enemies vulnerable to crits (and a great many are immune to them so its a dangerous route)...
sneaking PAST ENEMIES is what I said is broken, it means you don't get the rewards of KILLING those enemies and as such are too low level to even sneak anymore.

Sneak attacks requires that you play a class that gives those and it is just not enough damage to compensate for how situational it is how easy it is to lose it.

Greater invisibility is way better for that then the hide skill (in NWN)

A LOT of modules award almost no battle XP and reward huge amounts of XP for picking locks, disabling traps, interaction with npc, and questing. In most of these, is actually easier to get higer leveled with a sneaking rogue than simply poking sqisies with a sharp stick. Most of these modules also doesn't have huge mounts of SA-immune enemies, exactly because, in nwn, sneaking generally means rogue.

Plus, trueseeing beats invisibility. Nothing beats maxed skills+high Dex. I'm playing a ranged sneaking Rogue/Shadowdancer in HotU and I have never been detected, unless I bumped into a enemy. SA-immune enemies are annoying as hell, tough, but even then, rapidfire+HiPS is OP and I can manage very well.

This is ONLY in custom modules, the op explicitly asked about the official campaigns and expansions. Furthermore the awards for picking locks and disabling traps are usually trivial (IIRC a few of the expansions have those but are too insignificantly low) and are not in any way barred to someone who doesn't use sneak attacks. Several sorc/wizard familiars are rogues several levels higher then the player which can be summoned for such uses (and then sent back) and even my fighter can put 2 points each (cross class) for 1 rank each of open lock and disable device and take 20 (automatically done out of combat)... on a rare occasion he will need to use thieves tools or put on an item that gives a huge bonus to lockpicking or disable device.

Also the kill XP simply massively outweighs the XP for traps and locks.

Interaction with NPCs and Questing is available to ALL classes and does not give any one class a leg up over the other. Trueseeing is pretty damn rare. Much rarer then enemies immune to sneak attacks. And if they are immune to sneak attacks who cares if you can sneak by unseen? You still have to attack them for the loot and XP at which point you are no longer hidden and they can hit back.

What I was trying to state is that sneaking characthers works well in modules made with stealth in mind. The OC wasn't made with a focus on stealt or social skills, but on fights, therefore, these behaviors doesn't work well there. In some custom modules, the way XP is awarded is modified so questing an interaction award much more xp than killing. The point is exactly that if most of the XP avaliable through these means, it doesn't matter in which way you solve the problem, as long as you solve it. That way, characters focused on social skill or stealth won't be behind.

As far as the focus is on OC, minmaxing yourself a massive destroyer is kind of a wasted effort anyway, when you can pass it just fine with just about any failed character you want.

I could see this paying off in multiplayer, or on some specific modules. but generally crazy high combat skills are unnecessary.

Actually, the couple of times I've made a cheesy character have felt like cheating.
Weaponmaster with half a dozen better critical feats, that takes down demons and dragons in 1-2 rounds,
or a shadowdancer with massive sneak attack and couple of regeneration items.

Kind of almost kind of takes all the fun out.

My first character in NWN1 (I knew nothing about DnD at the time) was a wizard 1/sorc 1/bard 1.
It was made of epic fail.

I agree.
There are some truly well done modules out there with focuses that differ wildly than that of the OC