Interesting question. If I think about it, I would say: using a noun as an adjective may come across as more humorous, informal, crude/simple, or illustrative than using an adjective. But it's tricky because using a noun as an adjective in a lot of cases won't make sense. If you have any particular examples, please send them.
"He is a man child" vs "He is a childlike man" The first is more humorous and more 'mean' than the second one. The adjective in this case is more purely descriptive. It's more crude, but it's also more descriptive. Also, we invert the word order in this case, with 'child' coming after 'man.'
"He is a bear man" vs "He is a bearlike man." "Bearish" could be used in place of "bearlike" as an adjective. Same with this example...although we don't invert the word order, although you could say 'man bear.' 'Bearlike' is more plainly descriptive whereas calling someone a 'bear man' is more humorous or crude.
Also, it can be less precise and based on context. If you said "He is a dog man" that could mean he is a man who likes dogs or it could mean he is a man that looks/acts like a dog. Whereas, saying he is a 'doggish' or 'doglike' man would describe his appearance or behavior more precisely.