I think the different between redis and webserver like nginx is that all the operations in redis is almost the same, it is about less than 1ms. However the request to nginx fall in a widely range, some request need 10ms, while some request need 10s. Since nginx need do some file operations.

So the single model work well for redis, but it doesn't work well for nginx, since if there is a request in nginx that is blocking for about 10s, people can't tolerate this situation.