Bugs - see linked Lexicon article for breakdown or the github page. Alphabetical order not "importance" order.

• EffectAbilityDecrease - Won't "cache" more effects when it hits 3 ability score, so some are lost if some of the ability scores are removed later
  • This might be "intentional" due to how the game has a limit to amount of effects and some keep adding ability score changes (eg on hit properties)...
• EffectACIncrease and EffectACDecrease - various bugs, especially Decrease - https://github.com/Beamdog/nwn-issues/issues/266
• EffectBeam - apparently has some issues displaying the beam in some cases - https://github.com/Beamdog/nwn-issues/issues/435
• EffectDominated - NPC dominating an NPC is buggy
• EffectDispelMagicBest - no feedback sometimes https://github.com/Beamdog/nwn-issues/issues/452
• EffectEntangle - Various bugs, especially on overlapping removal and when the concentration check occurs (it occurs on spells with Concentration set to 0).
• EffectImmunity - accepting value 33 (minimally buggy though)
• EffectModifyAttacks - Egad where to start? What behaviour should it do. Stacking of positives and negatives. Recalculation bugs.
  • At least fixing the recalculation bugs which is something that shouldn't happen
  • A new effect to mimic a proper attack progression might be a better thing
• EffectPolymorph - Can a fix be in place to stop spells being lost due to stat-increasing items being equipped?
• EffectDamageReduction - DAMAGE_POWER_ENERGY buggy - essentially is +6
• EffectDarkness - Problems with Trueseeing
• EffectDazed - No statescript firing for some reason (means it's harder to reuse for other effects like Mesmerize or whatever)
• EffectTurnResistanceDecrease / EffectTurnResistanceIncrease - Does not affect PCs (but should do since they can technically polymorph into any race, and can have it on their hides even)

Various missing EFFECT_TYPE integers; EffectCutsceneDominated, EffectKnockdown and a few others. See https://nwnlexicon.com/index.php?title=Effect_type / https://lexicon.nwn.wiki/index.php?title=GetEffectType

These may be considered a pain :

• EffectDamageShield - Doesn't apply vs. alignment/racial type properly (but really should)
• EffectSpellResistanceIncrease - Doesn't work with vs. alignment (or race)
• EffectSanctuary and Etherealness are both massively weird in regards to even having Trueseeing. Really needs a think about.
  • Basically True Seeing should see through both fully properly but this is a gameplay change...
• EffectSleep - Possibly fix the wrong perception stuff - where they can't listen but can see!
  • Might have to be a ruleset.2da change
• EffectDisappearAppear - corpse doesn't appear if in flying mode

A "egad why do you care" stuff because it's funny:

• VersusTrapEffect - simply doesn't do anything!

Fixed in Preview

• EffectDamage - Bugs around nDamagePower. New parameter to retain previous effects might be best? A limited gameplay change in base game though since not many spells use it. https://github.com/Beamdog/nwn-issues/issues/265
  • Fixed - now properly works in spells. Doesn't really affect default spells (only Evards Tentacles really)
• EffectDamageResistance - can't have all the damage types concatenated which is a shame - https://github.com/Beamdog/nwn-issues/issues/106
  • Now works up to the maximum (can't be done with the max value)
• EffectDamageIncrease - fix in ruleset.2da FIX_EFFECTDAMAGEINCREASE_BYPASSING_DR_AND_DI
• EffectVisualEffect - Bug with re-applying and some other oddities around placeables; https://github.com/Beamdog/nwn-issues/issues/140 - fixed in preview
• EffectMovementSpeed - 2 bugs: https://github.com/Beamdog/nwn-issues/issues/239 - fixed plus some ruleset.2da entries added
• EffectNegativeLevel - At least the bonus HP parameter is bugged and needs fixing. Would be nice if spell uses were removed properly too but mmm. Some options around how the negative levels work in ruleset.2da might be nice (it's really overzealous on some and not so much other stats). - fixed no ruleset.2da entries yet though
• EffectRegenerate - Either stop negative values or code it properly so it kills the creature when they reach 0 - capped at 0
• EffectSpellFailure - Only applies last value, if cleared it resets to 0 rather than checking other effects of the same type, not magical by default - fixed in preview, stacks
• EffectTemporaryHitpoints - Wrong creator set if not a creature - fixed in preview
• EffectDamageDecrease - bugs with criticals: https://github.com/Beamdog/nwn-issues/issues/268 - fixed in preview
• EffectDeaf - Arcane spell failure instead of proper spell failure, or better yet one that only affects verbal spells without the silent metamagic - changed in preview to be 20% spell failure on non-silenced verbal spells. Doesn't stack with spell failure, highest is taken (20% or more).
• Some regressions with visual effects - specifically Beams: https://github.com/Beamdog/nwn-issues/issues/459 - Fixed in preview