It’s an odd comparison: both DF and RimWorld are more complex than each other here :)

RimWorld simulates organs in a more complete way than Dwarf Fortress does, and lets you surgically remove and replace those organs. That’s pretty cool, and a lot fun. Each part has hit points, and each can be lost in combat. A character without body parts suffers from infirmities or death, depending on how critical the part is. A character who is missing a leg can still function, but moves around more slowly. RimWorld tracks similar information about the body parts of animals and robots, which makes combat against them a lot of fun. Herbivores run away from pain, and robots still have organs like cameras and chemical reactors that can be destroyed to incapacitate them.

Dwarf Fortress simulates body parts in a more complete way than RimWorld does. Each body part (limb, torso, head, etc), has hair, skin, fat, muscle, and bone layers each with their own properties. These parts also have sensory and motor nerves as well as blood vessels. DF keeps track of the color of the skin and hair, the depth of the fat and muscle (and those are related to the Dwarf’s stats), and what type of damage each layer has taken (bruises, cuts, tears, etc). Damage to some layers can heal naturally, be surgically treated, get infected, leave a scar, etc. Others, like nerves, often won’t heal. This sometimes leaves characters unable to use body parts that they haven’t completely lost. It keeps track of some organs, such as the brain and the gonads, but not others like stomachs, hearts, lungs, etc. Surgical options are much more limited, as befits the setting. You can clean a wound (with or without soap), set bones, suture cuts and tears, and use casts or traction tables to immobilize broken limbs. A character who is missing a leg cannot walk normally, but the game will try its hardest to allow them some mobility using other parts, or you can give them crutches. Like RimWorld, Dwarf Fortress tracks the overall body plan for all species in the game (thousands upon thousands of them), as well as the unique information about each individual. Dwarf Fortress gains many of the same combat and storytelling benefits as RimWorld from its in–depth simulation.

Dwarf Fortress goes a step beyond RimWorld though, when it comes to Forgotten Beasts and the Undead.

Forgotten Beasts are horrific monsters left over from the chaotic creation of the world. They have _randomized_ body plans. Sometimes they will be a combination of two or more creatures, like a giant feathered iguana with bat wings and a fiery breath. Other times they will be a standard body plan (such as a colossal troll) made from an unusual substance (such as quartz, or fire, or iron). And it knows the material properties of quartz and fire and iron, and it can simulate how your weapons will do against them!

The Undead are a similar kind of fun. These are ordinary creatures (such as your own fallen dwarves) who have been reanimated by arcane forces. The Undead of course do not feel pain, do not need blood or intact nerves to use their body parts, and ignore all damage. And remember how a crippled character will still try to crawl if they are missing a leg? You don’t know fear until your dwarves are being chased by an undead skull that is pushing itself along by its eyebrows, biting anything that comes near. Your old nightmares will fade into irrelevance once one of your dwarves butchers a cow and the bloody skin and hair are reanimated to terrorize the innocent.