I wouldn't be surprised if readmes were rotten, but you should be able to rely on examples in docs and example programs in Cargo projects, because they are automatically built and run during unit tests.
That's interesting I didn't even think about that, then I'm not sure why it doesn't work. One case I still remember was where Actix (web framework) had docs pointing to severely older versions of the software, while the readme pointed to another, and none of them matched up quite right at the time... It was just not useful. If the examples work but you don't know where to go from there what can you even do at that point...
Actix had a major rewrite when it moved from old futures to new async/await model, so for a while there was a big difference between released/stable 0.x version and new shiny/dev 1.x version. Now it has mostly settled down.
I wouldn't be surprised if readmes were rotten, but you should be able to rely on examples in docs and example programs in Cargo projects, because they are automatically built and run during unit tests.