I think it is interesting to compare how D&D-adjacent games handle resistances and immunities. Skeletons and fire elementals are a good example; they can highlight if the game places focus on "Sorry, but you will have to try a different weapon/spell/power against this one enemy (and let us hope you are not are a fire elementalist with no fire-piercing up against a fire elemental)," or if the game would prefer to showcase other traits to distinguish enemies.

D&D 4e:
• Skeletons, as undead, have immunity to disease and poison, resist necrotic X, and vulnerable radiant X.
• Fire elementals have no special defenses against fire. Taking cold damage prevents them from shifting (moving safely).

Pathfinder 2e:
• Skeletons have void healing, inverting much (but not all) of the healing or damage they take from void and vitality abilities. Skeleton monsters have: Immunities bleed, death effects, disease, mental, paralyzed, poison, unconscious; Resistances cold X, electricity X, fire X, piercing X, slashing X.
• Fire elementals have: Immunities bleed, fire, paralyzed, poison, sleep; Weaknesses cold X.

Draw Steel:
• Skeletons, as undead, reduce incoming corruption or poison damage by X. (Void elementalists and undead summoners run into this.)
• An elemental crux of fire reduces incoming fire damage by X. (Fire elementalists have trouble.)

ICON:
• As of 2.0, the Relict (undead) have no special defenses that they gain simply by being Relict.
• As of 1.5, Ifrit elementals have no special defenses against fire.

13th Age:
• As of the 2e GM book, skeletons have resist weapons 16+ until at half HP. Weapon attacks that roll natural 15 or below deal half damage.
• As of 13 True Ways, fire elementals have resist fire 18+.

Daggerheart:
• Neither skeletons nor fire elementals have special defenses that they gain simply by being such.

How do enemy-specific resistances and immunities (or lack thereof) work in your own game? Do you prefer that they not exist?