What are the advantages and disadvantages of mixing the bypass flow with the core flow?
In simple terms, mixing the exhaust flow (with any bypass ratio) is good for all the reasons except for construction and weight. It is aerodynamically more efficient; it reduces noise (exhaust noise makes most of the jet noise); it cools combustion chamber (and other downwind parts) more efficiently, etc.

However, making the mixing chamber on a high bypass / big engine would require enormous outer nacelle to cover the whole engine (and extend somewhat beyond), and the weight penalty and complexity/cost negates all the benefits.