Candlekeep rats were given damage resistance in the original game purely to avoid the chunking animation, because it had no mechanical impact. They only have 1 hp, so damage resistance was not relevant. It was an aesthetic choice, not a matter of game balance. Obviously a rat does not have 90% resistance to being sliced open or bashed with a hammer.
With Legacy of Bhaal enabled, these rats give the player a completely unrealistic and frankly horrifying idea of what the difficulty has in store for them. Without knowing the exact numbers LoB gives you (it's not in the UI anywhere so most players will not) it's almost impossible to see those rats and not say "Well this is a complete waste of time, am I seriously grinding damage on rats? Time to do literally anything else".
I'll use shank as an example of what I mean. The +80 part of the LoB equation has the largest impact on low hp characters like him. He has 92 hp, or 23 times his normal hp. He doesn't take 23 times as long to kill, because you're almost guaranteed to get overkill damage on him (a 16 damage hit would only do 4 effective damage on him at his original hp, if you hit for 10 damage on average you'll kill him in around 10 hits instead of 1), but that gives you a good idea of the "worst case scenario" the mode will have in store for you. He can be pulled outside the room to fight the guards, and generally shouldn't be too difficult or time consuming to deal with. Ditto for his partner in crime. They clearly demonstrate that you're not in kansas anymore, but they aren't a slog.
The rats are far more extreme. They have 83 hp instead of 1. This means they take potentially 83x as long to kill. In addition, their damage resistance ensures that overkill damage is minimal (On a crit you can potentially deal 3 points of damage with a strong enough character/weapon). Without the damage resistance, the rats would be the ultimate display of how ludicrously supercharged enemies are in this mode, taking several big hits to kill 1 hp rats. With the damage resistance they're an obscene slog, absorbing over 4000 damage between them before finally being put down. That's like fighting dozens of copies of the candlekeep assassins.
To give an idea of how ludicrous the rats truly are, the final boss of the game has 163 hp by default, which means he has 569 hp on LoB mode. He has no damage resistance, so that's just his hp. Those rats have an effective hp of 830, or nearly 50% more durable than the final boss. Each. There are 5.
The rats cannot be dragged out of the building, and are nearly impossible to miss (my offhand weapon missed on something like a 4 when i was neither proficient with the weapon, nor dual wield giving close to a -10 to hit). It's simply a matter of zoning in and out repeatedly, dealing one combat rounds worth of damage at a time so you don't waste any more time than you have to on reloading from lucky crits.
It takes far longer to clear out the rats than it does to clear the entire first outdoor area, very likely longer than the first 2-3 areas combined (you're dealing over 4150 damage to kill the rats, that's alot of 91 hp xvarts) and the rats are one of the first encounters in the game. You're guaranteed to break multiple weapons over the course of the fight (I broke 5, I think) and it gives a terrible impression of the difficulty setting.
Legacy of Bhaal is a great addition, and I'm looking forward to continuing my run (The ogre is bugged and not getting his LoB hp), but the vast majority of players are going to hit those rats and think "THIS is what they were spending so much time and energy developing? This is boring and terrible!".