For the items, etc. the ones I can remember offhand are:

• Boots of Stormy Clamour (applies Reverberation with conditions) - never seen it proc with any of the conditions I've tried. Meanwhile, Diadem of Arcane Synergy will (likely incorrectly) proc on damn near anything, even self-buffs.
• Jhannyl's Gloves (auto-cures Poison/Paralyze/Blindness) - hasn't worked for me once. The +1 to saving throws is active though.
• Goad (disadvantage on attacks, from Battle Master subclass) - either doesn't work or is very inconsistent.
• Uncanny Dodge (Rogue) - I think everyone that's played a Rogue knows this one. It's got a weird passive effect implementation that auto-disables a lot.
• Chromatic Orb - having occasional issues getting Storm Sorcerer's Heart of the Storm to proc with this. Still pinning this one down, if anyone else has run into it?
• Lucky (feat) - doesn't reroll incoming crits.
• Polearm Master (feat) - I have no idea where the bonus action is. Either I'm blind or it's just missing.

There's also stuff on the plus side, like Titanstring Bow double dipping with Lightning Charges or, famously, Haste and Haste-like effects (Elixir of Bloodlust) granting second attacks on each additional action for lv. 5+ martial classes. It's possible Larian just balanced it this way, but I don't think so. It's crazy broken.