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?

Correction: Fire elementalists in Draw Steel have fire-piercing by level 2. Void elementalists and undead summoners still run into complications against undead, though.

>>95971657 (OP)
I'm a GURPSfag, so we got Damage Resistance (subtracts directly from basic damage to determine penetrating damage) and Damage Reduction (divides penetrate damage after Damage Resistance to determine actual injury).
Total damage immunity for most damage types (Burning, Corrosion, Crushing, Cutting, Impaling, and Piercing damage) does not exist. Not even fire elementals have total immunity to fire damage. They only have DR 10 against Heat/Fire. A typical mundane fire only deals 1d-1 damage, though, so this is usually enough for functional immunity.
It's technically possible to achieve functional immunity against physical damage without any DR. Insubstantiality with Reflexive can make you untouchable any time you're attacked; so long as the attack doesn't have Affects Insubstantial or otherwise transcend cross-dimensional barriers. Unkillable 2, Regeneration (Extreme), and Immunity to Unconsciousness can keep you fighting even when you're at -10xHP.
Immunity to damage types that only affect living things (Fatigue and Toxic) is possible, and is relatively cheap for only 30 XP. Machines and Undead typically have Immunity (Metabolic Hazards), Injury Tolerance (Unliving) which is pretty much Damage Reduction (vs. Impaling and Piercing damage, due to fewer biological vulnerabilities), Damage Resistance, and extra HP to represent their durability.

>>95972903
Additionally, against a human opponent, you can make Targeted Attacks to all sorts of hit locations for special effects like increased Wounding Modifiers, penalties to Knockdown rolls, crippling injuries, and so on. Nonhuman opponents might have different hit locations, or totally lack some. For instance, a homogeneous golem made of stone might lack a brain or heart to target for bonus damage, or blood to bleed from a cutting/impaling/piercing attack.
I think the closest thing you get to that is the immunity that constructs and undead in D&D 3.5e have to critical hits and sneak attack. Except critical hits and sneak attack exist in GURPS, but still work against machines and undead, because those are different mechanics.

I avoid using immunities unless unavoidable for whatever reason. Players are of course free to purchase them.

>>95973288
Same, except for the player usability bit. The only immunities I would consider on a permanent basis are one's that allow a clear means of dealing with an environment that a reasonably well-equipped person would have. So things like breathing underwater are fine, as well as high-altitude tolerance; and temporarily much more extreme immunities are fine.

My reasoning is these broad immunities (like a fire damage immunity) are uninteresting from your typical combat perspective; once anyone figures out that fire doesn't hurt you, their only real response is to stop using fire. A resistance leads to a more nuanced situation of the attacker determining whether they'd prefer a reliable attack with a reliable automatic bit of limited counterplay by the target, or if they would rather gamble on a less-reliable, unorthodox attack that should bypass resistance.

>>95973504
Fortunately there are far more ways of defeating an opponent than dealing damage, and everyone has access to them.