Access to financial services through a mobile phone, known as Mobile Financial Services (MFS), creates an opportunity to expand the reach of financial services to the 1.7 billion unbanked adults worldwide. Nevertheless, MFS adoption has been inconsistent, which motivates a need to identify the challenges that MFS users confront in different countries. In this work, we explore the Twitter as a potential data source to understand such challenges. More broadly, we assess whether (and how) publicly available Twitter data can augment the findings of expensive, large-scale research studies on MFS barriers. Our Qualitative Content Analysis of 9,000 mobile money grievance tweets that were extracted from 54 MFS customer care twitter feeds across six countries reveals service and access issues, incorrect transactions, and fraud as three main challenges MFS users report on Twitter. We discuss the nuances around these challenges and the substantial differences between the common issues reported in different countries. Ultimately, we conclude that Twitter data can elucidate the challenges of MFS adoption and also that it can augment the results of other types of MFS studies.