Searching with MeSH makes your search more relevant than searching with keywords (aka "All Fields" in PubMed) alone. However, there is a delay in getting MeSH terms added to articles, so if you only search using MeSH you will miss the latest research.
Also, there's not a MeSH term for every concept. Sometimes keywords are the most direct way to find what you need.
For a comprehensive search, ensure that you use a combination of MeSH and keywords.
Keyword search tools:
This video describing PICO was developed for the CINHL nursing database, but the concepts also apply to PubMed. Framing your question in PICO format can help you identify key components of your search.
PubMed and Ovid MEDLINE have nearly 100% of the same content, but how you search them is quite different. You can learn more about searching Ovid MEDLINE from UBC Library's tutorials.