Thank you for the opportunity to help you with your question!
One type of study where mixed method is not preferable is a study that
involves a smaller sample size population and that is needed faster.
Mixed method takes a lot of time as it addresses the concerns form all
angles and thus not preferable for such studies. The best method to use
for such thus is the qualitative approach.
Please let me know if you need any clarification. I'm always happy to answer your questions.