Actually a picture of a disc brake really didn't help me at all to understand the workings of a roller brake. Nor did a picture of a hub with no brakes at all. Coincidentally, neither did a bunch of posts about the quality or application of said roller brakes. Lets face it, no brake works perfect in every application. We all know this.
Kingfish actually helped answer the literal question for me quite directly with all that needed to be said after I simply clicked on the link to the information from the guru of bicycles.
You gave some really good input to the question "How well does a roller brake work?", though, had that been asked.
The OP asked about the availability of a disc-brake Nexus 3; hence, that picture of a disc-equipped Nexus 3. The OP asked if a roller brake can be added to any hub (specifically a coaster hub); the picture of a hub with "no brakes at all" was shown so the OP could see what was required on a hub to be rollerbrake compatible. (Those little splines are only on some Shimano Nexus and Nexave hubs; SRAM's site claims that some of their IGHs are designed to be Shimano roller brake compatible, too, but I haven't come across any yet.) And, in post #10, the OP
does ask directly for opinions on how well the roller brakes work.
When a waitress asks you "how does your lunch taste?", do you explain to her that you chew on the sandwich and nerve-endings on your tongue process various flavors which your mind analyzes regarding the flavor? Or do you offer an opinion, such as "pretty good" or "lousy"? (I haven't ever encountered a waitress that asked me "How well does your lunch taste"...)
Fact is, the OP
did ask about compatability, and many of us offered info on that. And, opinions are to be expected almost everywhere, especially on a forum called "Bike Talk", where someone asked about how a bike product works. You're not the OP and, oh! I'm sorry, what was
your contribution to this thread? No information at all, and some complaints about the presence of relevent opinions on a thread about rollerbrakes. Congrats.