Business Rule Framework is used heavily at SAP for their rapid application development. Proof can be found when you want to implement new functionality in S/4 HANA.
Most businesses running SAP do not realise that this tool can be extremely useful for their custom development, saving time and money. As soon as this opportunity becomes clear, then they seek BRF+ experts.
It is correct that BRF+ knowledge is a niche, and many SAP clients are not aware this functionality exists. But when they are actively looking for people with this skill than you can benefit by being one of the few that promote this capability on your resume.
Read the blog for Career Opportunities.