In BG2 Tactics mod (Sword Coast Stratagems), they identified that sneak backstab multipliers were incredibly powerful, so they nerfed it by allowing enemy mages / priests to cast Detect Illusion / Detect Invisibility / Invisibility Purge / True Seeing / True Sight if they suspected an invisible character were nearby. They were scripted to do it automatically based on your proximity if they were able to cast one of those spells, and most packs of dangerous hostiles would have a mage or priest, so you had to get in there quickly and backstab & run unless you cheesed it with the Cloak of Non-Detection.
If I remember correctly, the Tactics mod also allowed you to turn on pre-buffing for enemies, so even if you got a backstab in, you'd better hope the mage / priest was dead or they'd have Stoneskin / Protection from Magical Weapons and a whole lot of other stuff up instantly.
