Generally, words which formed in the shadow of a power imbalance are a bad choice. The n-word you used was one of contempt specifically for subjugated people of African heritage. To use the word to address black people today is to remind them that their race once literally owned. It's especially bad when the person using the term is white and thus echoing actual historical context. Black people addressing each other in this way perhaps consider it acknowledgment of a shared heritage and contemporary struggle against the very real relics of that history. People of other backgrounds using it can be seen to be appropriating that. The r-word you used is rooted in times where society locked up people who had delayed or injured development as they were deemed a burden, as defective and even less than human. Even today the word isn't addressed to people as a compliment. Using the word pejoratively, even for situations or objects, can remind people with developmental delays that they are still considered less than. Hearing it can erode already shaky self-esteem and confidence. People with delays don't carry around a neon sign to make sure you know that they're there, so it's often an invisible disability. It's best to just not go there. Calling someone an asshole isn't the same thing. But consider that "ass" is often considered swearing over here. Stirring up anguish over wage inequality also isn't the same thing. Nobody likes having to tip or and even less having to depend on tipping. Swearing/cussing isn't the same thing, and context is important. Some parts of the country are a lot more religious than you're probably used to, and their interpretations of how this should be applied to language is probably very different.
Some black people can be seen having great discontent over other black people using the word. Ken Carter in the movie Coach Carter forced his students to address each other in a respectful manner, dropping their then-usual "nigga", because the word shackles them again, makes them slaves, and Coach Carter believed they weren't slaves, so they shouldn't speak of themselves as such.Black people addressing each other in this way perhaps consider it acknowledgment of a shared heritage and contemporary struggle against the very real relics of that history.