The weaknesses of the MMR are as follows:

• A single researcher may have difficulty in carrying out both qualitative and quantitative research, especially if two or more approaches are expected to be done concurrently (i.e., the study might require a research team).
• The researcher has to learn about multiple methods and approaches and understand how to combine them appropriately.
• Methodological purists contend that one should always work within either a qualitative or quantitative paradigm.
• It is more expensive.
• It is more time-consuming.
• Some of the details of mixed research
  remain to be worked out fully by research methodologists

The other options reflect the strengths of mixed methods research.