Valkinaz: Main problem with both OC and MotB is its hate for "critical hits"
Darvin: Yes, they definitely went overboard with the undead. Thematically it made sense, but gameplay-wise... undead cause problems for certain common builds. The fact that such a ubiquitous creature type is outright immune to such a common game mechanic is a flaw with 3.5 in general (a lesson learned in later editions, where critical hit immunity is rarely seen).
I'm not even sure it makes sense thematically; I think the developers were just afraid of critical hits. (Consider the fact that, in 3.5, the developers made Keen and Improved Critical not stack, even though it seems logical that they should.) Then again, I grew up with JRPGs like Dragon Quest and Final Fantasy, in which anything could be critically hit (except for the PC in DQ1).
In Dragon Quest, not only could anything be critically hit (including Metal Slimes), but critical hits ignore defense, allowing them to easily kill Metal Slimes and their cousins. (For anyone not familiar with the DQ series, Metal Slimes have absurdly high defense (to the point where your physical attacks do only 0 or 1 damage), are immune to pretty much everything that isn't a physical attack, and tend to run away; however, they only have 3 or 4 HP (though their cousins in later games may have a few more), and give a lot of XP when defeated.)
In the original Final Fantasy, critical damage is added after defense is applied, and after the game sets the damage to the minimum of 1. As a result, even flans (enemies that look something like slimes) can be killed by critical hits, and later in the game, most attacks from fighter-types (especially Monks, which are *really* powerful at higher levels in FF1) will critical at least once. (Normally, you are supposed to use magic against Flans, but critical hits can also kill them.)
Darvin: Yes, they definitely went overboard with the undead. Thematically it made sense, but gameplay-wise... undead cause problems for certain common builds. The fact that such a ubiquitous creature type is outright immune to such a common game mechanic is a flaw with 3.5 in general (a lesson learned in later editions, where critical hit immunity is rarely seen).
Valkinaz: It woudn't be such problem if they took some stuff from supplementary books and kinda tweaked base class accordingly. Like pooling rogue variant from Unearthed Arcana which let him trade sneak dice for bonus fighter feat with choise menu in the start of module(similar to how wizard choose spell specialisation) and forcing player in OC to choose this variant over standart while using standart rogue class for enemies(in I remember correctly all bandits in game are rogue/fighter, rogue/cleric or rogue/mage proper dualclasses.
Or Neval at the start of undead fest could just bring few "specially enchanted" amulets that would let crit and sneak undead while they are in partymember inventory
Or, alternatively, not include a mechanic that makes certain character builds useless against certain enemy types. (There are other problematic mechanics, like magic immunity, that should also be avoided for similar reasons.)
Or, as yet another alternative, use a system that allows one to change their setup at any time. For instance, if fighting undead, the player could just switch out the thief build for a cleric build that's focused on killing undead, and then switch back to a thief build when fighting enemies that critical hits work well against.